procmail is an email server software component — specifically, a message delivery agent (MDA).

procmail reads mail messages given to it on standard input, delivering or otherwise dispositioning each one.

Rather, some other mail program will call upon procmail to deliver a message according to the user's wishes.

Message transfer agents (MTAs), such as Sendmail or Postfix), can be configured to use procmail to deliver all mail.

procmail's behavior is controlled by a config file (by default, .procmailrc in the user's home directory) containing one or more recipes, read in order.

By default, procmail will "bounce" undeliverable mail back to the sender, or discard it entirely.

For approximately ten years, procmail was not maintained, and multiple serious security vulnerabilities[7] were discovered in the intervening time span[3] (since fixed).