diff --git a/message/lib.php b/message/lib.php
index 84d1f040f99..81f7a911c17 100644
--- a/message/lib.php
+++ b/message/lib.php
@@ -967,6 +967,7 @@ function message_post_message($userfrom, $userto, $message, $format, $messagetyp
             $tagline = get_string('emailtagline', 'message', $SITE->shortname);
 
             $messagesubject = message_shorten_message(strip_tags($message), 30).'...';
+            $messagesubject = str_replace("\n", ' ', $messagesubject);  // make sure it's all on one line
 
             $messagetext = format_text_email($message, $format).
                            "\n\n--\n".$tagline."\n"."$CFG->wwwroot/message/index.php?popup=1";