A Mail Server is a computer system that sends and receives e-mail. In most cases, web servers and mail servers are combined into a single machine. However, major "Internet Service Providers" and general email services (like Gmail and Hotmail) may use specialized hardware to send and receive email.

For a computer system to function as a mail server, it must include mail server software. This software allows the system administrator to create and manage email accounts for any domain hosted on the server (host).

Mail servers send and receive e-mail using standard e-mail protocols. For example, the SMTP protocol sends messages and processes outgoing mail requests. The IMAP and POP3 protocols receive messages and are used to process incoming mail. When you log into a mail server using a webmail interface or email client, these protocols handle all connections.

Mail server software is available for multiple platforms. The most popular mail server for Windows is Microsoft Exchange Server, an enterprise product used by large businesses. On the Linux operating system, Exim for sending mail and Dovecot and Courier for receiving mail are the most popular.