Sendmail

What Sendmail does:

• Listen on network ports for mail.
• Sort mail and deliver it locally or externally to other servers.
• Append mail to files or pipe it through other programs.
• Queue mail (if immediate delivery fails).
• Convert email addresses to/from user names, or handle mailing lists.
• Reads rules for special mail handling, so it can try to catch spam, or check for correctness.