domain without mail support bouncing internal mail with "unknown user"

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.

Any ideas?

Please tell us what exact sender and recipient addresses you used, and please post a log excerpt from /var/log/mail.log at the point of failing delivery attempt. Enclose it in [code][/code] tags to preserve linebreaks. Please also post the contents of the file /etc/postfix/virtual (you can edit it later, since it contains all your email addresses).