Relay Access Denied - 2008 Version

Whenever I try to send mail to any of the sites I've set up with VirtualMin I get several errors one of which is "relay access denied". I have checked everything but I am guessing I've missed something. Can anyone give me a few places to look at? Below is part of the text from the error. I send an email to one of my other email accounts on another server and when I replied to the message, I got the error

----- The following addresses had permanent fatal errors ----- <beverly@mydomain.com> (reason: 554 5.7.1 <beverly@mydomain.com>: Relay access denied) ----- Transcript of session follows ----- ... while talking to mail.mydomain.com.: >>> DATA <<< 554 5.7.1 <beverly@mydomain.com>: Relay access denied 554 5.0.0 Service unavailable <<< 554 5.5.1 Error: no valid recipients Final-Recipient: RFC822; beverly@mydomain.com Action: failed Status: 5.7.1 Remote-MTA: DNS; mail.mydomain.com Diagnostic-Code: SMTP; 554 5.7.1 <beverly@mydomain.com>: Relay access denied Last-Attempt-Date: Sat, 15 Nov 2008 08:14:12 -0500 (EST)