Thanks for the advice clacour, I will surely be checking into PAM a little bit more now.
You're right that for most intents and purposes checking mail is low-intensity, but where I work (I'm a sys admin for a highly technology based firm) is kind of a special case. We have under 100 employees, but we generate a MASSIVE amount of email. I mean massive!
To help with the load, as well for other reasons beyond the scope of this thread, we split up the email service from one machine that was handling incoming and outgoing mail to 2. One of these handles outgoing mail and one handles incoming mail, spam/virus checking, and runs the pop services. Spam checking and pop logins are where the real strain on the machine comes from, sendmail alone is pretty efficient.
As for the machine itself its a dual cpu Athlon MP1800+ with 2GB of RAM, which is currently doing ok with the load that is on it. Things do get pretty bogged down though when a lot of activity starts up. A large majority of our email is generated by applications, so if there is a flurry of events in our business these scripts can generate a lot of messages in a very short time. To combat this some I wanted to use time restrictions to cut down on the average load on the machine so that these peaks wouldn't be such a problem.... something 2 minutes would be fine for me, 1 minute between updates is a bit much though.
Anyway, I'm babbling about stuff I'm sure you don't want to hear, so thanks for the help!