Domain of sender address ... does not exist

[Philip A. Prindeville]

I'm occasionally seeing the following error:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  philipp... at redfish-solutions.com
    (generated from xyzzy at users.sourceforge.net)
    SMTP error from remote mail server after RCPT TO:<philipp... at redfish-solutions.com>:
    host mail.redfish-solutions.com [66.232.79.143]:
    553 5.1.8 <philipp... at redfish-solutions.com>... Domain of sender address philipp... at redfish-solutions.com does not exist


This is on an externally generated email that is coming into my domain
(redfish-solutions.com). The mailbox name is valid (it's been munged
here to protect against spam address harvesters).

Looked at the cf/cf/generic-linux.cf briefly, but it's been too long
since I've munged .cf files (these days I cop out and us the .mc
versions instead).

What's going on, and how do I fix it?

Thanks,

-Philip