sendmail rejecting bad domains

[Gene McCulley <mcculley@cuspy.com>]

I have someone trying to send me email from a machine that is behind a 
firewall.  The firewall relays the mail through, but leaves the
sender's machine name on the message.  The sender's machine is not in
the DNS, so sendmail rejects it with "501 Sender domain must exist".
The sender can't fix his corporation's broken firewall.  How do I get
sendmail to allow this message through?  I care less about spam than I 
do lost email.

Thanks for any clues.

-- 
|     Gene McCulley     |      mcculley@cuspy.com     | Voice: (407) 265-0772 |
| Cuspy Solutions, Inc. | http://cuspy.com/~mcculley/ |  Fax: (407) 265-0773  |
|       Your father was a hamster and your mother smelt of elderberries!      |