fix(smtp): strip a folded Bcc header from the outgoing DATA

lib/src/private/smtp/commands/smtp_sendmail_command.dart

@@ -73,6 +73,14 @@ class _SmtpSendCommand extends SmtpCommand {
   }
 }
 
+/// The `Bcc` header line and every folded continuation line under it.
+///
+/// `Header.render` folds a value longer than
+/// `MailConventions.textLineMaxLength` onto `\r\n\t`-prefixed lines, which
+/// three or four addresses already do. Matching only the first physical line
+/// left the rest of the list in the DATA that every To/Cc recipient received.
+final _bccHeader = RegExp(r'^Bcc:.*\r\n(?:[ \t].*\r\n)*', multiLine: true);
+
 /// Sends a MIME message
 class SmtpSendMailCommand extends _SmtpSendCommand {
   /// Creates a new DATA command
@@ -82,10 +90,7 @@ class SmtpSendMailCommand extends _SmtpSendCommand {
     List<String> recipientEmails, {
     required bool use8BitEncoding,
   }) : super(
-         () => message.renderMessage().replaceAll(
-           RegExp('^Bcc:.*\r\n', multiLine: true),
-           '',
-         ),
+         () => message.renderMessage().replaceAll(_bccHeader, ''),
          from?.email ?? message.fromEmail,
          recipientEmails,
          use8BitEncoding: use8BitEncoding,
@@ -104,10 +109,7 @@ class SmtpSendMailDataCommand extends _SmtpSendCommand {
     List<String> recipientEmails, {
     required bool use8BitEncoding,
   }) : super(
-         () => data.toString().replaceAll(
-           RegExp('^Bcc:.*\r\n', multiLine: true),
-           '',
-         ),
+         () => data.toString().replaceAll(_bccHeader, ''),
          from.email,
          recipientEmails,
          use8BitEncoding: use8BitEncoding,

test/smtp/smtp_sendmail_command_test.dart

@@ -0,0 +1,63 @@
+import 'package:enough_mail/enough_mail.dart';
+import 'package:enough_mail/src/private/smtp/commands/smtp_sendmail_command.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('SmtpSendMailCommand', () {
+    test('strips a Bcc header that was folded onto continuation lines', () {
+      final message =
+          (MessageBuilder()
+                ..from = const [MailAddress('Me', 'me@example.com')]
+                ..to = const [MailAddress('You', 'you@example.com')]
+                ..cc = const [MailAddress('Carol', 'carol@example.com')]
+                ..bcc = [
+                  for (var i = 0; i < 6; i++)
+                    MailAddress(
+                      'Hidden Recipient Number $i',
+                      'hidden.recipient.$i@example.com',
+                    ),
+                ]
+                ..subject = 'Hello'
+                ..addTextPlain('Body'))
+              .buildMimeMessage();
+      final rendered = message.renderMessage();
+      expect(
+        rendered,
+        contains('\r\n\t'),
+        reason: 'the fixture must actually fold the Bcc header',
+      );
+
+      final command = SmtpSendMailCommand(
+        message,
+        null,
+        message.recipientAddresses,
+        use8BitEncoding: false,
+      );
+      final data = command.getData();
+
+      expect(data, isNot(contains('Bcc')));
+      expect(data, isNot(contains('hidden.recipient')));
+      expect(data, isNot(contains('\r\n\t')));
+      expect(data, contains('To: "You" <you@example.com>\r\n'));
+      expect(data, contains('Cc: "Carol" <carol@example.com>\r\n'));
+      expect(data, contains('Subject: Hello\r\n'));
+      expect(data, contains('Body'));
+    });
+
+    test('keeps every recipient in the envelope', () {
+      final message =
+          (MessageBuilder()
+                ..from = const [MailAddress('Me', 'me@example.com')]
+                ..to = const [MailAddress('You', 'you@example.com')]
+                ..bcc = const [MailAddress('Hidden', 'hidden@example.com')]
+                ..subject = 'Hello'
+                ..addTextPlain('Body'))
+              .buildMimeMessage();
+
+      expect(message.recipientAddresses, [
+        'you@example.com',
+        'hidden@example.com',
+      ]);
+    });
+  });
+}