brief Overview of Linux Mail Server in General

Message Transfer

The 3 parts to message transfer are MTA, MDA and MUA.
1. Mail Transfer Agent - (examples: sendmail,postfix,qmail,exim) handles distributing outgoing mail and listening for incoming mail from internet
2. Mail Delivery Agent - Accepts message from MTA and copies the message into a user's mailbox. Example is procmail
3. Mail User Agent - Supports POP3 and IMAP features to get and read email from server. Browser based- squirrelmail, horde, atmail, roundcube etc. PC based - Outlook, Eudora, Thunderbird, Kmail etc, Text Based- mutt, pine, mailx etc.

Email Flow

1. The sender uses a Mail User Agent (MUA) to compose an email. An MUA, often referred to as a email client, is a program that allows a user to compose, send and receive email.
2. The mail is sent to a Mail Transfer Agent (MTA) which is responsible for sending the email to the receipient's MTA. An MTA transfers mail messages between servers via the SMTP protocol.
3. The recipient's MTA receives the email and passes it on to a Mail Delivery Agent (MDA). An MDA manages the user's mailbox and handles mails for delivery to the MUA using either the POP or IMAP protocol. POP (Post Office Protocol) - POP is generally used to download all messages from the mailbox, store them on the user's PC as new messages and delete them from the server. IMAP (Internet Message Access Protocol) - Allows users to view their mailboxes on the server and to delete mail only when told to do so.
4. The recipient uses an MUA to check and retrieve messages from the MDA.