Mail server Overview:

Send mail is the most popular UNIX-based implementation of the Simple Mail Transfer Protocol (SMTP) for transmitting e-mail.

The Mail Transfer Agent (MTA) is responsible for moving email between different email servers using the Simple Mail Transfer Protocol (SMTP), Examples of UNIX MTAs include sendmail, postfix, Exim and qmail.

The message store is a data repository. A user queries this repository using a Mail User Agent (MUA) such as Eudora or Netscape. 

Email Protocols and some of the basic components

MUA: Mail User Agent          The email application that a user sends/receives (thunderbird,pine,outlook) MTA: Mail Transport Agent: transports email messages between hosts using SMTP. A message may involve several MTAs as it moves to its intended destination.  Such as (sendmail,postfix,qmail) MDA: Mail Delivery Agent The server agent that accepts email from MTA, and places into users mailbox (procmail) Mail Access Protocols: There are two primary protocols used by email client applications to retrieve email from mail servers: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP). SMTP: MUAs and MTAs use this protocol for sending emails Mail delivery from a client application to the server, and from an originating server to the destination server POP3: Post Office Protocol (Ver 3) MUAs use this protocol for receiving their emails from the final server IMAP: Internet Message Access Protocol     MUAs can use this protocol to send and receive emails on the servers Simple Scenario: Sending and receiving e-mail locally This scenario demonstrates how e-mail is processed between local users. Jawed is using the Netscape mail client. He writes a message and sends it to Hamid@mycompany.com. The following figure illustrates the path the mail message takes through the network.