Just ran across one i have not seen before, so here it is. Testing out a mailserver and trying to find out how and why stuff is bouncing. Here is the message i was seeing when telneting onto port 25 from the outside AND the inside:
brain:/home/tek# telnet mail.example.com 25
Connected to mail.example.com.
Escape character is '^]'.
220 mail.example.com ESMTP Postfix (Debian/GNU)
501 5.1.7 Bad sender address syntax
This always worked in the past, not sure when but at some point there is another config variable i had implemented to filter spam and now its causing me grief with a few clients.