I cannot reproduce this with current roundup head.
The problem looks like a bug in a custom detector. To further debug this
(if the problem persists) you can apply the following patch to
roundup/mailgw.py to get a traceback in the error mail -- feel free to
reopen the issue if you get a traceback from code in roundup core not in
one of the detectors:

--- a/roundup/mailgw.py
+++ b/roundup/mailgw.py
@@ -1158,7 +1158,7 @@ Mail message was rejected by a detector.
             raise MailUsageError, _("""
 There was a problem with the message you sent:
    %(message)s
-""") % locals()
+""") % locals() + '\n' + traceback.format_exc ()
 
         return self.nodeid