[X Servers] Mail rejections

[Wally Rodriguez]

We are experiencing a problem where our mail server (OSX 10.2.6) is
rejecting mail from only one domain (bellsouth.net).

The error the senders are receiving is:   (my comment in brackets)

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"The destination mail system [our server] rejected your return address.

Solution: Verify that the return e-mail address you are using is correct.
The return address was <nameofsender at bellsouth.net>"

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Does anyone know why this could be happening?

I have checked all the relay settings and even rebuilt the server database
from scratch, but I'm still having the problem, a huge one for us as we're
in Florida, where most of our clients use bellsouth.net

Thanks in advance for your replies