- Patch #234403 by Gabor, Bevan and me: drupal_mail_send() had problems with linebreaks headers.

includes/mail.inc

@@ -184,12 +184,12 @@ function drupal_mail_send($message) {
     return mail(
       $message['to'],
       mime_header_encode($message['subject']),
-      // Note: e-mail uses CRLF for line-endings, but PHP's API requires LF
-      // for the message body.
+      // 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']),
-      // For headers, PHP's API requires that we use CRLF normally.
-      join("\r\n", $mimeheaders)
+      // For headers, PHP's API suggests that we use CRLF normally,
+      // but some MTAs incorrecly replace LF with CRLF. See #234403.
+      join("\n", $mimeheaders)
     );
   }
 }