Index: includes/mail.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/mail.inc,v
retrieving revision 1.20
diff -u -p -r1.20 mail.inc
--- includes/mail.inc	7 May 2009 10:41:12 -0000	1.20
+++ includes/mail.inc	7 Jun 2009 15:36:26 -0000
@@ -183,7 +183,9 @@ function drupal_mail_send($message) {
     }
     return mail(
       $message['to'],
-      mime_header_encode($message['subject']),
+      // The email might not be sent properly if the subject field
+      // contains newline characters. see http://drupal.org/node/84883.
+      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']),