Are there any scripts or services that anyone would recommend to monitor mail queues and send out alarms for weirdness?

On another note, I thought that it was fairly obvious from the stuffed up mail queue who was sending spam. However, I've recently had a person swear up and down that they didn't do it. So much so that I actually believe them.

So...how do you track down a spam email in the mail queue to a particular user account in a shared hosting environment? Apparently it's not as easy as I think it is.