One of the first things you should do is take a look at your maillog, you'll be doing this a lot
CODE
/var/log/maillog
and you might want to follow it from time to time. . .
CODE
tail -f /var/log/maillog
Another simple and very useful tool little I've found is Sendmail stats. It's small, simple and to the point:
http://www.reedmedia.net/software/sendmail.../#other-reports
it'll give you a detailed report of what's going on in your maillogs.
Also, let's say you've discovered that dreaded sm-acceptingconnectings mailscanner queue / cache bug has thousands of spam messages looking to be cleared, flushed or otherwise deleted so that they don't keep mailing out.
For Parallels / Ensim, you want to check in
/home/virtual/FILESYSTEMTEMPLATE/services/sendmail/mqueue.scanned
/home/virtual/FILESYSTEMTEMPLATE/services/sendmail/mqueue
For massive amounts of spam just waiting to flow out.
That way, once you plug the holes, you can also get rid of some of those files if you need to.
Just a few basics you might want to keep handy, I'd love to hear others add to it