Procmail is one of the more popular automated mail processors. It is fairly powerful and complex, so if you want to be a power user, you'll have to read the documentation:
Procmail is a program which enables you to filter incoming mail. You can destroy unwanted mail without having to read it, send automatic replies (including the "vacation" message that tells people you are away from your mail), and lots more.
People often ask how to avoid receiving "spam" mail, or how to bounce mail from someone who is annoying them. These pages tell you how to install procmail; you can then tailor it to do all those things, or whatever else you want.
Some of the other things you can do with procmail (such as vacation mail auto-replying) are set up for you in the sample "Main" file in the basic procmail installation; just follow the instructions in that file to enable the facility. If you want to get even more complicated, you should slog through the procmail manpages.
If you're trying to catch mail from certain users and file it in a different folder, take a look at this example.
Remember that you can post questions to the newsgroup concordia.dept.iits.help. Frequently-requested services may be added to "Main", if time permits.
A note to offsite users who wish to use these recipes: you're welcome to do so, but of course we disclaim any responsibility for the consequences. Enjoy.
Thanks to Neil Schwartzman for volunteering as a guinea pig to evaluate this Web page and the sample procmail set-up itself; he tripped on the problems so you won't have to. I hope.