=== modified file 'modules/system/mail.sending.inc'
--- modules/system/mail.sending.inc	2009-10-16 04:00:03 +0000
+++ modules/system/mail.sending.inc	2009-11-06 13:30:37 +0000
@@ -47,7 +47,9 @@ class DefaultMailSystem implements MailS
     }
     return mail(
       $message['to'],
-      mime_header_encode($message['subject']),
+      // The email might not be sent properly if the subject field contains
+      // newline characters.
+      str_replace("\n", ' ', mime_header_encode($message['subject'])),
       // Note: e-mail uses CRLF for line-endings, but PHP's API requires LF.
       // They will appear correctly in the actual e-mail that is sent.
       str_replace("\r", '', $message['body']),