Sendmail configuration problem? Some servers refusing to receive mail from me...

["Donald Tyler" <dtyler frazerbilt com>]

I have setup a sendmail server and have been using it successfully for a
week or so now. I added some more account and these people and been
complaining of having some of their emails returned to them with the
following error:

------------------------------------------------------------------------
----

   ----- The following addresses had permanent fatal errors -----
<***** letsos com>
    (reason: 501 5.5.4 Invalid Address)

   ----- Transcript of session follows -----
... while talking to mail.letsos.com.:
>>> HELO mail.frazerbilt.com.
<<< 501 5.5.4 Invalid Address
554 5.0.0 Service unavailable

----------------------[ here is the details.txt file:
----------------------

Reporting-MTA: dns; mail.frazerbilt.com.
Received-From-MTA: DNS; adsl-65-66-208-5.dsl.hstntx.swbell.net
Arrival-Date: Thu, 1 Jan 2004 09:45:24 -0600

Final-Recipient: RFC822; *****letsos.com
Action: failed
Status: 5.5.0
Diagnostic-Code: SMTP; 501 5.5.4 Invalid Address
Last-Attempt-Date: Thu, 1 Jan 2004 09:45:25 -0600

------------------------------------------------------------------------
----

The * is to hide the real email address.

I thought it might have something to do with the reverse lookup of my
mail servers name, but I checked and it is correctly mapped to
mail.frazerbilt.com.

My servers IP address is 65.45.50.130.

Can anyone tell me what exactly that error message means?

Thanks