I have a domain setup without mail support. That domain uses Gmail with appropriate DNS setup and works fine. Mail generated on the server sent to any user of that domain fails with “unknown user” (externally sent mail works fine).
In other words, mail is being bounced and not sent externally to Gmail as if mail for the domain were configured, but it isn’t.
I tried enabling/disabling mail on the domain to see if that would change anything and it didn’t.
The domain in question is the hostname. This is a Linode running CentOS6. DNS is external (Linode).
This is a recent reinstall, migrating from CentOS5 to CentOS6.