I use the POP3 Service that comes with Windows 2003 and that requires that I allow SMTP Anonymous Authentication. Although I've turned off relaying, I would like to do is reject any commands that would try to relay, and only allow the ones that are trying to deliver mail.
There are no special commands that tell the SMTP server to relay. The way it determines if it needs to relay is if the domain name is different.
So when you relay you'll still use the MAIL FROM: & RCPT TO: etc commands as normal.
so mail sent to you @ domain.local would look like
While mail that is trying to relay to domain.remote would look like this: