How to Email Works

Using email is very easy, but setting up your own email server is not. This article describes how email works to give you an introduction on the various components needed to implement your own mail server.

1. The sender uses a Mail User Agent (MUA) to compose an email. An MUA,  
    often referred to as a mail 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 computers via the SMTP protocol. Postfix, Sendmail, Exim and
    Qmail are examples of an MTA.

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)
  Although most mail clients have an option to Leave a copy on the server, 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.
  
  Mbox and Maildir are two common mailbox formats used in Unix. Dovecot, Cyrus and Courier are examples of an MDA.
  
  4. The recipient uses an MUA to check and retrieve messages from the MDA.