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'System undeliverable' messages

I am running a small Win2003, Exchange 2003 environment. I have SMTP enabled because some of the users need POP3 access. I have also followed Microsoft guidelines to secure relaying. I get regular "system undeliverable" messages for mail that are supposed to have originated from within an account in my domain, but was not sent by anyone from the domain. So relaying of some form is still occurring. How can I stop this?
These NDRs are likely the result of spoofed messages; that is, they are messages sent from another server that are spoofing your e-mail address (or e-mail addresses from your organization). You can get third-party content filtering software that can block these bogus NDRs, but often they block valid NDRs, too, so that might not be a viable option for you.

The good news is that relaying is probably not occurring. The bad news is that you may have to deal with these for a while until you devise a way to filter them at your e-mail gateway.

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