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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.

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up vote 0 down vote accepted

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

RCPT TO: someone@domain.local
MAIL FROM: someone@sending.domain
Subject: Subject
<Message Body>

While mail that is trying to relay to domain.remote would look like this:

RCPT TO: someone@domain.remote
<if you have relay restriction in place you'll get a relay denied message here>
MAIL FROM: evildoer@sending.domain
Subject: Subject
<message Body>
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The only case you don't receive a relay denied message is when you try a command like this: RCPT TO: <""> These end up in the Drop folder. – Josh Sep 14 '09 at 22:08

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