Add BillingEmail field for commercial customers; support comma-separated multi-email

- Customer entity + DTO: new BillingEmail field (accounting/invoicing address)
- Email fields now accept comma-separated lists; DTO validates each address individually
- NotificationService: SendToEmailListAsync helper fans out to all addresses in a list;
  NotifyQuoteSentAsync accepts optional overrideEmail so staff can send to an ad-hoc address
- Migration: AddCustomerBillingEmail
- Customer Create/Edit/Details views updated to show Billing Email field
- customer-billing-email.js: client-side helpers for billing email input

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Application/DTOs/Customer/CustomerDtos.cs

@@ -9,6 +9,7 @@ public class CustomerDto
     public string? ContactFirstName { get; set; }
     public string? ContactLastName { get; set; }
     public string? Email { get; set; }
+    public string? BillingEmail { get; set; }
     public string? Phone { get; set; }
     public string? MobilePhone { get; set; }
     public string? Address { get; set; }
@@ -52,10 +53,13 @@ public class CreateCustomerDto : IValidatableObject
     public string? ContactLastName { get; set; }
 
     [Display(Name = "Email")]
-    [EmailAddress(ErrorMessage = "Please enter a valid email address")]
-    [StringLength(200)]
+    [StringLength(1000)]
     public string? Email { get; set; }
 
+    [Display(Name = "Billing / Accounting Email")]
+    [StringLength(1000)]
+    public string? BillingEmail { get; set; }
+
     [Display(Name = "Phone")]
     [Phone(ErrorMessage = "Please enter a valid phone number")]
     [StringLength(20)]
@@ -143,6 +147,33 @@ public class CreateCustomerDto : IValidatableObject
                 "Please provide at least one contact method (Email or Phone)",
                 new[] { nameof(Email), nameof(Phone) });
         }
+
+        // Validate each address in comma-separated email fields
+        foreach (var addr in SplitEmails(Email))
+        {
+            if (!IsValidEmail(addr))
+                yield return new ValidationResult(
+                    $"'{addr}' is not a valid email address.",
+                    new[] { nameof(Email) });
+        }
+        foreach (var addr in SplitEmails(BillingEmail))
+        {
+            if (!IsValidEmail(addr))
+                yield return new ValidationResult(
+                    $"'{addr}' is not a valid email address.",
+                    new[] { nameof(BillingEmail) });
+        }
+    }
+
+    private static IEnumerable<string> SplitEmails(string? value) =>
+        string.IsNullOrWhiteSpace(value)
+            ? []
+            : value.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+    private static bool IsValidEmail(string email)
+    {
+        try { _ = new System.Net.Mail.MailAddress(email); return true; }
+        catch { return false; }
     }
 }
 

src/PowderCoating.Application/Interfaces/INotificationService.cs

@@ -9,7 +9,7 @@ public interface INotificationService
     /// Notify when a quote is created/sent. Handles both registered customers and prospects.
     /// Optionally attaches the quote PDF to the email.
     /// </summary>
-    Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null);
+    Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null, string? overrideEmail = null);
 
     /// <summary>
     /// Sends the quote approval link to the customer via SMS.
@@ -58,7 +58,7 @@ public interface INotificationService
     /// Notify customer when an invoice has been sent.
     /// Optionally includes an online payment link in the email body.
     /// </summary>
-    Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null);
+    Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null, string? overrideEmail = null);
 
     /// <summary>
     /// Notify customer (internal) when a payment has been recorded on an invoice.

src/PowderCoating.Core/Entities/Customer.cs

@@ -6,6 +6,7 @@ public class Customer : BaseEntity
     public string? ContactFirstName { get; set; }
     public string? ContactLastName { get; set; }
     public string? Email { get; set; }
+    public string? BillingEmail { get; set; }  // Accounting/invoicing email for commercial customers
     public string? Phone { get; set; }
     public string? MobilePhone { get; set; }
     public string? Address { get; set; }

src/PowderCoating.Infrastructure/Migrations/20260507224749_AddCustomerBillingEmail.cs

@@ -0,0 +1,71 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace PowderCoating.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddCustomerBillingEmail : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "BillingEmail",
+                table: "Customers",
+                type: "nvarchar(max)",
+                nullable: true);
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 7, 22, 47, 45, 755, DateTimeKind.Utc).AddTicks(648));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 7, 22, 47, 45, 755, DateTimeKind.Utc).AddTicks(653));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 7, 22, 47, 45, 755, DateTimeKind.Utc).AddTicks(655));
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "BillingEmail",
+                table: "Customers");
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 6, 19, 59, 56, 264, DateTimeKind.Utc).AddTicks(846));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 6, 19, 59, 56, 264, DateTimeKind.Utc).AddTicks(852));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 6, 19, 59, 56, 264, DateTimeKind.Utc).AddTicks(853));
+        }
+    }
+}

src/PowderCoating.Infrastructure/Services/NotificationService.cs

@@ -70,7 +70,7 @@ public class NotificationService : INotificationService
     ///   - Customer: respects NotifyByEmail; still writes a Skipped log if opted out.
     /// Always writes a NotificationLog row so the Notifications Sent tab shows delivery history.
     /// </summary>
-    public async Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null)
+    public async Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null, string? overrideEmail = null)
     {
         try
         {
@@ -80,7 +80,8 @@ public class NotificationService : INotificationService
             // Prospect quote (no customer record yet)
             if (quote.CustomerId == null)
             {
-                if (string.IsNullOrWhiteSpace(quote.ProspectEmail))
+                var prospectEmail = !string.IsNullOrWhiteSpace(overrideEmail) ? overrideEmail : quote.ProspectEmail;
+                if (string.IsNullOrWhiteSpace(prospectEmail))
                     return;
 
                 var prospectName = !string.IsNullOrWhiteSpace(quote.ProspectContactName)
@@ -97,7 +98,7 @@ public class NotificationService : INotificationService
                 var plainText = StripHtml(fullHtml);
 
                 var (success, error) = await _emailService.SendEmailAsync(
-                    quote.ProspectEmail, prospectName, subject, plainText, fullHtml,
+                    prospectEmail, prospectName, subject, plainText, fullHtml,
                     pdfAttachment, pdfFilename, "application/pdf",
                     replyToEmail, replyToName);
 
@@ -107,7 +108,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.QuoteSent,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = prospectName,
-                    Recipient = quote.ProspectEmail,
+                    Recipient = prospectEmail,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -124,7 +125,11 @@ public class NotificationService : INotificationService
 
             var customerName = GetCustomerDisplayName(customer);
 
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            // Override address (ad-hoc staff entry) takes priority over customer record.
+            var emailToUse = !string.IsNullOrWhiteSpace(overrideEmail) ? overrideEmail : customer.Email;
+            var quoteEmails = ParseEmailList(emailToUse);
+            // Bypass NotifyByEmail preference when staff explicitly supplies an override address.
+            if ((customer.NotifyByEmail || !string.IsNullOrWhiteSpace(overrideEmail)) && quoteEmails.Count > 0)
             {
                 var baseUrl = await GetBaseUrlAsync();
                 var values = BuildQuoteSentValues(companyName, customerName, quote, baseUrl);
@@ -135,8 +140,8 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, baseUrl);
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
-                    customer.Email, customerName, subject, plainText, fullHtml,
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
+                    emailToUse, customerName, subject, plainText, fullHtml,
                     pdfAttachment, pdfFilename, "application/pdf",
                     replyToEmail, replyToName);
 
@@ -146,7 +151,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.QuoteSent,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -156,10 +161,10 @@ public class NotificationService : INotificationService
                     CompanyId = quote.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (quoteEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.QuoteSent,
-                    customerName, customer.Email, quote.CompanyId, customerId: customer.Id, quoteId: quote.Id));
+                    customerName, string.Join(", ", quoteEmails), quote.CompanyId, customerId: customer.Id, quoteId: quote.Id));
             }
         }
         catch (Exception ex)
@@ -195,7 +200,9 @@ public class NotificationService : INotificationService
 
             if (quote.CustomerId == null)
             {
-                // Prospect — use ProspectPhone; no opt-in check (they explicitly provided a phone)
+                // Prospect — requires explicit staff-recorded consent (TCPA compliance)
+                if (!quote.ProspectSmsConsent)
+                    return (false, "SMS consent has not been recorded for this prospect. Edit the quote to record verbal consent before sending via SMS.");
                 smsPhone = quote.ProspectPhone;
                 if (string.IsNullOrWhiteSpace(smsPhone))
                     return (false, "No phone number on file for this prospect.");
@@ -279,7 +286,8 @@ public class NotificationService : INotificationService
             var (replyToEmail, replyToName) = await GetEmailFromAsync(quote.CompanyId);
             var customerName = GetCustomerDisplayName(customer);
 
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            var approvedEmails = ParseEmailList(customer.Email);
+            if (customer.NotifyByEmail && approvedEmails.Count > 0)
             {
                 var values = new Dictionary<string, string>
                 {
@@ -295,7 +303,7 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, await GetBaseUrlAsync());
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
                     customer.Email, customerName, subject, plainText, fullHtml,
                     replyToEmail: replyToEmail, replyToName: replyToName);
 
@@ -305,7 +313,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.QuoteApproved,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -315,10 +323,10 @@ public class NotificationService : INotificationService
                     CompanyId = quote.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (approvedEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.QuoteApproved,
-                    customerName, customer.Email, quote.CompanyId, customerId: customer.Id, quoteId: quote.Id));
+                    customerName, string.Join(", ", approvedEmails), quote.CompanyId, customerId: customer.Id, quoteId: quote.Id));
             }
         }
         catch (Exception ex)
@@ -349,7 +357,8 @@ public class NotificationService : INotificationService
                 : NotificationType.JobStatusChanged;
 
             // Email for all status changes
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            var statusEmails = ParseEmailList(customer.Email);
+            if (customer.NotifyByEmail && statusEmails.Count > 0)
             {
                 // ScheduledDate is when the shop plans to complete the work;
                 // DueDate is the customer deadline — fall back to it only when no scheduled date is set.
@@ -377,7 +386,7 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, await GetBaseUrlAsync());
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
                     customer.Email, customerName, subject, plainText, fullHtml,
                     replyToEmail: replyToEmail, replyToName: replyToName);
 
@@ -387,7 +396,7 @@ public class NotificationService : INotificationService
                     NotificationType = notifType,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -397,10 +406,10 @@ public class NotificationService : INotificationService
                     CompanyId = job.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (statusEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.JobStatusChanged,
-                    customerName, customer.Email, job.CompanyId, customerId: customer.Id, jobId: job.Id));
+                    customerName, string.Join(", ", statusEmails), job.CompanyId, customerId: customer.Id, jobId: job.Id));
             }
 
         }
@@ -427,7 +436,8 @@ public class NotificationService : INotificationService
             var customerName = GetCustomerDisplayName(customer);
 
             // Email
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            var completedEmails = ParseEmailList(customer.Email);
+            if (customer.NotifyByEmail && completedEmails.Count > 0)
             {
                 var values = new Dictionary<string, string>
                 {
@@ -444,7 +454,7 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, await GetBaseUrlAsync());
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
                     customer.Email, customerName, subject, plainText, fullHtml,
                     replyToEmail: replyToEmail, replyToName: replyToName);
 
@@ -454,7 +464,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.JobCompleted,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -464,10 +474,10 @@ public class NotificationService : INotificationService
                     CompanyId = job.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (completedEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.JobCompleted,
-                    customerName, customer.Email, job.CompanyId, customerId: customer.Id, jobId: job.Id));
+                    customerName, string.Join(", ", completedEmails), job.CompanyId, customerId: customer.Id, jobId: job.Id));
             }
 
             // SMS — skip when the caller (Admin/Manager) will handle it via the compose modal
@@ -611,7 +621,7 @@ public class NotificationService : INotificationService
     /// (the <paramref name="paymentUrl"/> parameter). Without a payment URL the email is a
     /// standard "here is your invoice" message with no payment CTA.
     /// </summary>
-    public async Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null)
+    public async Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null, string? overrideEmail = null)
     {
         try
         {
@@ -622,7 +632,15 @@ public class NotificationService : INotificationService
             var (replyToEmail, replyToName) = await GetEmailFromAsync(invoice.CompanyId);
             var customerName = GetCustomerDisplayName(customer);
 
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            // Override email (staff-provided ad-hoc address) takes priority over customer record.
+            // Use BillingEmail when set (commercial accounting dept); fall back to primary Email.
+            var invoiceEmail = !string.IsNullOrWhiteSpace(overrideEmail)
+                ? overrideEmail
+                : (!string.IsNullOrWhiteSpace(customer.BillingEmail) ? customer.BillingEmail : customer.Email);
+
+            var invoiceEmails = ParseEmailList(invoiceEmail);
+            // Bypass NotifyByEmail preference when staff explicitly supplies an override address.
+            if ((customer.NotifyByEmail || !string.IsNullOrWhiteSpace(overrideEmail)) && invoiceEmails.Count > 0)
             {
                 var dueText = invoice.DueDate.HasValue
                     ? $" Payment is due by {invoice.DueDate.Value:MMMM d, yyyy}."
@@ -661,8 +679,8 @@ public class NotificationService : INotificationService
                     ? StripHtml(htmlBody) + $"\r\n\r\nPay online: {paymentUrl}"
                     : StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
-                    customer.Email, customerName, subject, plainText, fullHtml,
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
+                    invoiceEmail, customerName, subject, plainText, fullHtml,
                     pdfAttachment, pdfFilename, "application/pdf",
                     replyToEmail, replyToName);
 
@@ -672,7 +690,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.InvoiceSent,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -682,10 +700,10 @@ public class NotificationService : INotificationService
                     CompanyId = invoice.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (invoiceEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.InvoiceSent,
-                    customerName, customer.Email, invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
+                    customerName, string.Join(", ", invoiceEmails), invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
             }
         }
         catch (Exception ex)
@@ -710,7 +728,8 @@ public class NotificationService : INotificationService
             var (replyToEmail, replyToName) = await GetEmailFromAsync(invoice.CompanyId);
             var customerName = GetCustomerDisplayName(customer);
 
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            var paymentEmails = ParseEmailList(customer.Email);
+            if (customer.NotifyByEmail && paymentEmails.Count > 0)
             {
                 var balanceText = invoice.BalanceDue > 0
                     ? $" Remaining balance: {invoice.BalanceDue:C}."
@@ -733,7 +752,7 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, await GetBaseUrlAsync());
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
                     customer.Email, customerName, subject, plainText, fullHtml,
                     replyToEmail: replyToEmail, replyToName: replyToName);
 
@@ -743,7 +762,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.PaymentReceived,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -753,10 +772,10 @@ public class NotificationService : INotificationService
                     CompanyId = invoice.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (paymentEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.PaymentReceived,
-                    customerName, customer.Email, invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
+                    customerName, string.Join(", ", paymentEmails), invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
             }
         }
         catch (Exception ex)
@@ -782,7 +801,8 @@ public class NotificationService : INotificationService
             var (replyToEmail, replyToName) = await GetEmailFromAsync(invoice.CompanyId);
             var customerName = GetCustomerDisplayName(customer);
 
-            if (customer.NotifyByEmail && !string.IsNullOrWhiteSpace(customer.Email))
+            var reminderEmails = ParseEmailList(customer.Email);
+            if (customer.NotifyByEmail && reminderEmails.Count > 0)
             {
                 var dueDate = invoice.DueDate.HasValue
                     ? invoice.DueDate.Value.ToString("MMMM d, yyyy")
@@ -806,7 +826,7 @@ public class NotificationService : INotificationService
                 var fullHtml = AppendUnsubscribeFooterHtml(htmlBody, customer.UnsubscribeToken, company, await GetBaseUrlAsync());
                 var plainText = StripHtml(fullHtml);
 
-                var (success, error) = await _emailService.SendEmailAsync(
+                var (success, error, recipientsLog) = await SendToEmailListAsync(
                     customer.Email, customerName, subject, plainText, fullHtml,
                     replyToEmail: replyToEmail, replyToName: replyToName);
 
@@ -816,7 +836,7 @@ public class NotificationService : INotificationService
                     NotificationType = NotificationType.PaymentReminder,
                     Status = success ? NotificationStatus.Sent : NotificationStatus.Failed,
                     RecipientName = customerName,
-                    Recipient = customer.Email,
+                    Recipient = recipientsLog,
                     Subject = subject,
                     Message = plainText,
                     ErrorMessage = error,
@@ -826,10 +846,10 @@ public class NotificationService : INotificationService
                     CompanyId = invoice.CompanyId
                 });
             }
-            else if (!string.IsNullOrWhiteSpace(customer.Email))
+            else if (reminderEmails.Count > 0)
             {
                 await WriteLog(SkippedLog(NotificationChannel.Email, NotificationType.PaymentReminder,
-                    customerName, customer.Email, invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
+                    customerName, string.Join(", ", reminderEmails), invoice.CompanyId, customerId: customer.Id, invoiceId: invoice.Id));
             }
         }
         catch (Exception ex)
@@ -1324,7 +1344,7 @@ public class NotificationService : INotificationService
         {
             var customer = invoice.Customer
                 ?? await _context.Customers.AsNoTracking().FirstOrDefaultAsync(c => c.Id == invoice.CustomerId);
-            if (customer == null || string.IsNullOrWhiteSpace(customer.Email)) return;
+            if (customer == null || ParseEmailList(customer.Email).Count == 0) return;
 
             var customerName = GetCustomerDisplayName(customer);
             var company = await _context.Companies.AsNoTracking()
@@ -1374,7 +1394,7 @@ Thank you for your business,
 <p style='color:#6b7280;font-size:14px;'>Thank you for your business,<br/><strong>{System.Net.WebUtility.HtmlEncode(companyName)}</strong></p>
 </div>";
 
-            await _emailService.SendEmailAsync(customer.Email, customerName, subject, plain, html,
+            await SendToEmailListAsync(customer.Email, customerName, subject, plain, html,
                 replyToEmail: replyToEmail, replyToName: replyToName);
         }
         catch (Exception ex)
@@ -1389,7 +1409,7 @@ Thank you for your business,
         {
             // Determine recipient — linked customer or prospect contact
             string? toEmail = quote.Customer?.Email ?? quote.ProspectEmail;
-            if (string.IsNullOrWhiteSpace(toEmail)) return;
+            if (ParseEmailList(toEmail).Count == 0) return;
 
             string customerName;
             if (quote.Customer != null)
@@ -1448,7 +1468,7 @@ Thank you for your business,
 <p style='color:#6b7280;font-size:14px;'>Thank you for your business,<br/><strong>{System.Net.WebUtility.HtmlEncode(companyName)}</strong></p>
 </div>";
 
-            await _emailService.SendEmailAsync(toEmail, customerName, subject, plain, html,
+            await SendToEmailListAsync(toEmail, customerName, subject, plain, html,
                 replyToEmail: replyToEmail, replyToName: replyToName);
         }
         catch (Exception ex)
@@ -1500,6 +1520,52 @@ Log in to your Stripe Dashboard to respond to this dispute. You typically have 7
         }
     }
 
+    /// <summary>
+    /// Parses a comma-separated email field into individual, trimmed addresses.
+    /// Silently ignores blank entries; does not validate format beyond requiring '@'.
+    /// </summary>
+    private static List<string> ParseEmailList(string? emailField)
+    {
+        if (string.IsNullOrWhiteSpace(emailField)) return [];
+        return emailField
+            .Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Where(e => e.Contains('@'))
+            .ToList();
+    }
+
+    /// <summary>
+    /// Sends the same email to every address in a comma-separated <paramref name="emailList"/>.
+    /// Returns (anySuccess, lastError, comma-joined recipient string for logging).
+    /// </summary>
+    private async Task<(bool Success, string? ErrorMessage, string RecipientsLog)> SendToEmailListAsync(
+        string? emailList,
+        string toName,
+        string subject,
+        string plainText,
+        string? htmlBody = null,
+        byte[]? attachmentData = null,
+        string? attachmentFilename = null,
+        string? attachmentContentType = null,
+        string? replyToEmail = null,
+        string? replyToName = null)
+    {
+        var emails = ParseEmailList(emailList);
+        if (emails.Count == 0) return (false, "No valid email addresses", string.Empty);
+
+        bool anySuccess = false;
+        string? lastError = null;
+        foreach (var email in emails)
+        {
+            var (ok, err) = await _emailService.SendEmailAsync(
+                email, toName, subject, plainText, htmlBody,
+                attachmentData, attachmentFilename, attachmentContentType,
+                replyToEmail, replyToName);
+            if (ok) anySuccess = true;
+            else lastError = err;
+        }
+        return (anySuccess, anySuccess ? null : lastError, string.Join(", ", emails));
+    }
+
     private static string GetCustomerDisplayName(Customer customer)
     {
         if (!customer.IsCommercial)

src/PowderCoating.Web/Views/Customers/Create.cshtml

@@ -74,7 +74,7 @@
                             </div>
                             <div class="col-md-6">
                                 <label asp-for="Email" class="form-label">Email <span class="text-danger">*</span> <span class="text-muted fw-normal">(required if no phone number)</span></label>
-                                <input asp-for="Email" type="email" class="form-control" placeholder="name@example.com" />
+                                <input asp-for="Email" type="email" multiple class="form-control" placeholder="name@example.com (comma-separate multiple)" />
                                 <span asp-validation-for="Email" class="text-danger"></span>
                             </div>
                             <div class="col-md-3">
@@ -87,6 +87,14 @@
                                 <input asp-for="MobilePhone" type="tel" class="form-control" placeholder="(555) 123-4567" />
                                 <span asp-validation-for="MobilePhone" class="text-danger"></span>
                             </div>
+                            <div class="col-md-6" id="billingEmailRow" style="display:none;">
+                                <label asp-for="BillingEmail" class="form-label">Billing / Accounting Email
+                                    <span class="text-muted fw-normal">(invoices sent here)</span>
+                                </label>
+                                <input asp-for="BillingEmail" type="email" multiple class="form-control" placeholder="accounting@company.com (comma-separate multiple)" />
+                                <span asp-validation-for="BillingEmail" class="text-danger"></span>
+                                <div class="form-text">When set, invoices are emailed here instead of the contact email.</div>
+                            </div>
                         </div>
                     </div>
 
@@ -372,4 +380,5 @@
 
 @section Scripts {
     <partial name="_ValidationScriptsPartial" />
+    <script src="~/js/customer-billing-email.js"></script>
 }

src/PowderCoating.Web/Views/Customers/Details.cshtml

@@ -97,11 +97,37 @@
                             <div class="col-md-6">
                                 <label class="text-muted small mb-1">Email</label>
                                 <p class="mb-0">
-                                    <a href="mailto:@Model.Email" class="text-decoration-none">
-                                        <i class="bi bi-envelope me-1"></i>@Model.Email
-                                    </a>
+                                    @if (!string.IsNullOrEmpty(Model.Email))
+                                    {
+                                        <a href="mailto:@Model.Email" class="text-decoration-none">
+                                            <i class="bi bi-envelope me-1"></i>@Model.Email
+                                        </a>
+                                    }
+                                    else
+                                    {
+                                        <span class="text-muted">Not provided</span>
+                                    }
                                 </p>
                             </div>
+                            @if (Model.IsCommercial)
+                            {
+                            <div class="col-md-6">
+                                <label class="text-muted small mb-1">Billing / Accounting Email</label>
+                                <p class="mb-0">
+                                    @if (!string.IsNullOrEmpty(Model.BillingEmail))
+                                    {
+                                        <a href="mailto:@Model.BillingEmail" class="text-decoration-none">
+                                            <i class="bi bi-envelope-at me-1"></i>@Model.BillingEmail
+                                        </a>
+                                        <span class="badge bg-info bg-opacity-10 text-info ms-2 small">Invoices</span>
+                                    }
+                                    else
+                                    {
+                                        <span class="text-muted">Not set — invoices go to contact email</span>
+                                    }
+                                </p>
+                            </div>
+                            }
                             <div class="col-md-6">
                                 <label class="text-muted small mb-1">Phone</label>
                                 <p class="mb-0">

src/PowderCoating.Web/Views/Customers/Edit.cshtml

@@ -78,7 +78,7 @@
                             </div>
                             <div class="col-md-6">
                                 <label asp-for="Email" class="form-label">Email</label>
-                                <input asp-for="Email" type="email" class="form-control" placeholder="name@example.com" />
+                                <input asp-for="Email" type="email" multiple class="form-control" placeholder="name@example.com (comma-separate multiple)" />
                                 <span asp-validation-for="Email" class="text-danger"></span>
                             </div>
                             <div class="col-md-3">
@@ -91,6 +91,14 @@
                                 <input asp-for="MobilePhone" type="tel" class="form-control" placeholder="(555) 123-4567" />
                                 <span asp-validation-for="MobilePhone" class="text-danger"></span>
                             </div>
+                            <div class="col-md-6" id="billingEmailRow" style="display:none;">
+                                <label asp-for="BillingEmail" class="form-label">Billing / Accounting Email
+                                    <span class="text-muted fw-normal">(invoices sent here)</span>
+                                </label>
+                                <input asp-for="BillingEmail" type="email" multiple class="form-control" placeholder="accounting@company.com (comma-separate multiple)" />
+                                <span asp-validation-for="BillingEmail" class="text-danger"></span>
+                                <div class="form-text">When set, invoices are emailed here instead of the contact email.</div>
+                            </div>
                         </div>
                     </div>
 
@@ -436,4 +444,5 @@
 
 @section Scripts {
     <partial name="_ValidationScriptsPartial" />
+    <script src="~/js/customer-billing-email.js"></script>
 }

src/PowderCoating.Web/wwwroot/js/customer-billing-email.js

@@ -0,0 +1,12 @@
+(function () {
+    var select = document.getElementById('IsCommercial');
+    var row = document.getElementById('billingEmailRow');
+    if (!select || !row) return;
+
+    function toggle() {
+        row.style.display = select.value === 'true' ? '' : 'none';
+    }
+
+    toggle();
+    select.addEventListener('change', toggle);
+})();