Relaying Exploit?

[Nico]

I just looked into a Spam issue and discovered that an account on a server can relay through another domain setup on the same server but can only send mail to domains hosted on the same server. Apparently this user, which is yet to be determined, is spamming other users on the server relaying through another domain on the server.

Is there a way to prevent this?

[snowgod]

Not currently, No.

Logs might reveal something about who it actually was. Check out exim_mainlog* (in /var/log).

You will need to know one of the actually addresses that got spammed though. Then just grep the logs.