Penguin

Exim is a mail program (technically I think it's both an MTA and an MDA). It is quite common, due to:

  1. being free software,
  2. having a reputation for being easy to set up and configure (compare and contrast with SendMail), and
  3. being the default mail handler for Debian.

Also see our EximNotes.

ActiveDirectoryMail is also in the pipeline.