1. The procmail program is used by Red Hat Linux for all local maildelivery. In addition to just delivering mail, procmail can be used for automatic filtering, presorting, and other mail handling jobs. Procmail is also the basis for the SmartList mailing list processor. From Redhat 8.0 RPM

2. Versatile e-mail processor. Can be used to create mail-servers, mailing lists, sort your incoming mail into separate folders/files (real convenient when subscribing to one or more mailing lists or for prioritising your mail), preprocess your mail, start any programs upon mail arrival (e.g. to generate different chimes on your workstation for different types of mail) or selectively forward certain incoming mail automatically to someone. From Debian 3.0r0 APT

3. autonomous mail processor From whatis