Email Server Setup - Basic Components
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An email server has several components. At the very least, it includes a Mail Transfer Agent (MTA) and a Mail Delivery Agent (MDA). These two components perform different yet necessary tasks. The Mail Delivery Agent hold emails for delivery to users. A Mail Transfer Agent, however, sends and receives email to and from other servers on the Internet.
There are some general, accepted standard protocols for doing these different tasks. For MTA, the standard for all servers on the Internet is SMTP (Simple Mail Transfer Protocol). This server takes mail from email clients and transfers it to whatever destination server it is bound for. For MDAs, the two accepted Internet standard protocols are POP (Post Office Protocol) and IMAP (Internet Mail Access Protocol).
POP is an older, very widely accepted MDA. When email is downloaded from a POP server, the mail client generally erases the mail messages from the server. In this paradigm, the user is responsible for storing email locally, and must maintain his or her own backups and organization structure. The other protocol, called IMAP, is designed to keep all mail on the email server, and places the responsibility for backup on the server operator, not the user. IMAP allows for users to create a remote directory structure for organizing mail. Currently, POP is falling out of style in favor of the more robust IMAP protocol.
While this distinction between MDA and MTA may seem simple and clear-cut, much of the complexity and difficulty inherent in setting up an email server arises from the places where these two server daemons interact and confusion that can arise at that point, as well as the strict security requirements for connecting an email server to the internet.
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