I've got a Debian Etch system running Exim4-daemon-heavy.
The system is open to the internet, but the intention is that it will only receive legitimate mail coming from a spam-filtering service, which runs as a proxy ahead of it. (I can't just limit access to those IPs though, because I do have some authorized users who relay via my server on port 25. I know I should be using 587 - but currently I'm not.)
The general way this works is:
[Internet] -> [SMTP proxy] -> [My Server]
Unfortunately I've got spammers sending mail directly to the mailserver, and ignoring the MX record(s). So it seems like my obvious solution is to either:
Add a header to each processed message at the SMTP proxy.
Add a header at my server for each incoming message unless the mail is coming from an authorized relayer. (ie. Somebody who has completed SMTP AUTH.)
That way I could use procmail to just junk messages that came direct, via senders who ignored my MX records.
I'm pretty sure that Exim4 could be coerced into adding a header such as "X-Submitter: $ip" - to record the remote IP which submitted the message, but I'm unsure how that should be done.
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