A mail server (sometimes called MTA – Mail Transport Agent) is an application that is used to transfer mails from one user to another. A mail server handles both sending and receiving mails using protocols such as SMTP (Simple Mail Transfer Protocol) or ESMTP (Extended SMTP) for sending mails and POP3 (Post Office Protocol 3) or IMAP (Internet Message Access Protocol) for receiving mails.
Four mail servers are most popular on Linux:
- Sendmail – the most widely used MTA on UNIX/Linux systems. It’s very powerful, but also difficult to configure.
- Postfix – designed as a modular replacement for Sendmail, rather than a single program that does everything. Postfix uses multiple programs, each of which handles its own specific small task. Postfix was designed to be easier to configure as well as more reliable and secure than sendmail, and it has become the default mail server on many Linux distributions (e.g. openSUSE).
- Exim – like Sendmail, Exim is a monolithic server (a single binary controls all the facilities of the MTA), but easier to configure.
- qmail – a modular server with security as the major design goal. It was designed to be lightweight, secure, and reliable.