Domain of Sender does not exist

[brad davison]

We have SMTP AUTH set up on our mail server.  Our mail server accepts mail 
for several domains.
x.com
y.com
z.com

Outgoing messages from [user]@x.com, and [user]@y.com goes out fine, but 
emails going out from [user]@z.com are getting:

Domain of Sender does not exist ([user]@z.com] 5.1.8

The oddest thing, is that this server is the MTA for z.com.  Sending to 
[user]@z.com will end up on this box no problem.

I did not set up our DNS, and it is in Active Directory.  In our DNS "z.com" 
in the Forward Lookup Zone has A record for the email server, and MX record 
for the email server.

What could be hanging this up?  The x.com and y.com are not specified in our 
DNS locally at all, so I am not sure how to model the DNS for these domains.

Also, when I 'paused' the z.com zone, I got the
Domain of Sender does not resolve 4.1.8 error.

Any leads on where my DNS or configs might be not able to resolve this 
domain?

Thanks
Brad

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