I set up a server with Postfix/Dovecot on Linode/Ubuntu 10.04 according to this guide, admittedly I've never done this before. Local mail works just fine, but trying to send email to it from external addresses results in errors like this:

Delivery to the following recipient failed permanently:

<address>@ni-mate.com

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 553 553 Unknown recipient. (state 13).

I honestly have no idea what to do or which configuration files/logs anyone needs to see.