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How can I tell if my Solaris server is being used as a mail relay

I suspect my Solaris 5.8 server is being used as a mail relay from an outside source, how can I tell?
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I have been trying to find a software tool to do the same thing, but have been unsuccessful.

If your server dose not need to do mail at all, then you could do an
/ete/init.d/sendmail stop

This will turn off all mail services on the server. (until you reboot)

You can also manually check the logs at /var/log/syslog looking for sendmail entries to see if there is much off site traffic.
grep sendmail /var/log/syslog |more

Easy, check the sendmail log files.  If you have a source that isn't from your domain or localhost and the destination is also not from your domain or localhost, then your server is being used as a relay.

I can't remember which version of sendmail comes in Solaris 8 but something to get you start:

Easy way - submit your email server IP to trusted mail relay checker, it will check your server for all known types of relay (including bounce relay and many other types, which are hard do discover manually).
Here are two trusted checkers (they will not disclose your IP, but they will include it in blacklist, so you will need to ask them again to recheck when problem will be fixed, if any):
Although the accepted answer is good, and those methods will tell you if there is a configuration error that allows relaying, they do not tell you if there is a compromised account that is being used to send spam.  Or worse if one of your own users is a spammer!

What would be really good is a tool that monitors mail flow and alerts the sysadmin if someone suddenly sends 50 or more messages in a short period of time.
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