The mail finally bounced back with the error message below. I searched the web and found you need to have your hostname and domain in the file /etc/mail/local-host-names (which already done).
----- The following addresses had permanent fatal errors -----
<user1 (at mark) domain.com>
(reason: 553 5.3.5 system config error)
----- Transcript of session follows -----
553 5.3.5 domain.com. config error: mail loops back to me (MX problem?)
554 5.3.5 Local configuration error
The below is what I received in testing by telneting to the port.
telnet x.x.x.x 25
helo user1 (at mark) domain.com
501 5.0.0 Invalid domain name
I have already stop/start sendmail (via the menu and shell). What's next?
Thanks.
Mike