The program "procmail" is a replacement for the local mailer (variously called /bin/mail, /usr/bin/mail, mail.local, rmail, etc...). It has been ported to run on virtually every Unix-like OS you're likely to run into, and has a whole host of features. It is typically about 30% faster performing the job of the local mailer than programs such as /bin/mail or /usr/bin/mail, it has been hammered on widely to make it extremely secure (much more so than most local mailers) and very robust. Procmail is also capable of helping you put a quota on a user's mailbox through the standard Unix quota mechanism.

In short, whatever you've got, you're almost guaranteed that procmail is better (if nothing else, the author has been able to focus lots of time and energy into making it the best and fastest tool available, while most system vendors just throw something together as fast as they can and move on to the whole rest of the OS).

However, this only begins to scratch the surface of what procmail is capable of. Its most important feature is the fact that it gives you a standard way to create rules (procmail calls them "recipes") to process your mail before the messages get put into your mailbox, and for that feature alone, it is one of the most important tools any administrator can have in their repertoire. By filtering out or automatically dealing with 80% of your daily, unwanted items, it lets you spend more time on the hard 20%.

Note that recent releases of version 8 sendmail natively support using procmail as an alternate local mailer (see "FEATURE(local_procmail)" for version 8.7 and above). They also support procmail as an additional local mailer, if you're concerned about flat-out replacing your current local mailer with procmail (see "MAILER(procmail)" in version 8.7 and above).

You can also install procmail as a user and run it out of your .forward file, although this tends to be a bit slower and less efficient.

More information about procmail can be found at https://www.procmail.org and the latest version can be found at that link as well.

Procmail is also the core to a mailing list management package called "SmartList", so if you've already got procmail, adding SmartList may be a good option. Some listowners prefer Majordomo, Listserv, or one of those other programs, but SmartList has more than a few adherents as well. Your personal tastes will dictate whether you swear by SmartList or at it.