Add missing quotes around datetime value.

Author: bramley

public_html/lists/admin/processbounces.php

@@ -247,7 +247,7 @@ function processBounceData($bounceid, $msgid, $userid, $bounceDate = null)
                         %d,
                         -1,
                         %d,
-                        %s
+                        "%s"
             )',
                 $tables['user_message_bounce'],
                 $userid, $bounceid, $bounceDate)