There are several different components to the email system. They work together to move, deliver or retrieve your email.

Mail Transfer Agent (MTA)

The MTA does a great deal of the hard work in moving mail around as it is responsible to move the mail from the local MTA to the destination MTA on the Internet. The Mail Transfer Agent works closely with DNS in making this all happen. The MTA uses a specific language SMTP to transfer mail on port 25, which is a standard. Several examples of MTAs are Sendmail, Postfix, and QMAIL.

Mail Delivery Agent (MDA)

The MDA will receive the mail destined for the local network from the MTA and then will make this mail available for the user. The MDA will use POP3 on port 110 or IMAP on port 143 to make this available to users. Examples of MDA are Dovecot or Cyrus-IMAP.

Mail User Agent (MUA)

The MUA is the client program that the end user uses to retrieve and view email. Users are able to view web based email with a browser but will user tools like Outlook, Thunderbird, Mutt, or Evolution to download mail to the local machine.

When you send or read your email, the only part you see is the MUA, which is a fancy way of saying email client. But, there’s a lot more than that involved. To send an email, you’ll first sit down at your computer and fire up Thunderbird, or whichever other email client that you’re using. When you compose the message and click on the send button, the MUA will send it to the MTA. (The MTA could either be on the corporate network or at your ISP.) This MTA will send the message to successive MTA’s until it gets to the MTA that serves the email recipient. This MTA will then send the message to a Post Office Protocol (POP)/Internet Mail Application Protocol (IMAP) server. This server will store the email until the recipient accesses it with her MUA. Of course, this same process could take place within a corporate network instead of across the Internet.