As of July 2012, the School of Math now supports a new email filtering language called Sieve. It is possible to write scripts for Sieve which will create mail filtering rules. Such rules can be used to sort incoming mail, intercept spam, and the like. More information on Sieve can be found on this site and in this PDF.
It is also possible to configure mail filters, as before, on mail.math.ias.edu. After logging in, select "Filters" on the left (under the "Mail" option), then click on the rule you wish to edit, or select "New Rule" to create a new mail rule. You can also set your vacation message by clicking on "Vacation" (also under "Mail" on the left) and entering the relevant information to display in your message.
There are also shell programs to facilitate the use of Sieve. For instance, typing "vacation" into the command line will bring up a prompt which provides an alternate means for setting a vacation message.
You can also configure mail delivery system is currently in use via the shell. Running "mailbox", with no other options, prints the current mail delivery settings. Entering "mailbox -x" or "mailbox --mdbox" will switch mail delivery to mdbox, "mailbox -p" or "mailbox --maildir" will choose maildir, and "mailbox -p" or "mailbox --mbox" will select the mbox/plain delivery method. In addition, the option -v (or --verbose) prints out what the mailbox program is attempting to do as it runs, and the option -d (or --debug) prints out debugging information as it runs. Please note that when running the mailbox script, you will be prompted for your password.
Also, please note that procmail and forward will no longer work with mdbox.