How E-Mail works

Email, as simple as it is to use, relies on a more complicated set of operating procedures than that of the Web. For most users, its operation is transparent, which means that it is not necessary to understand how email works in order to be able to use it.

However, the short introduction below has been provided to help users understand its basic principles, give them an idea of how to best configure their email clients and inform them about the underlying mechanisms of spam. 

 

Mail Transport Agent (MTA): a program which acts as a "mail server". Specifically, it's responsible for managing a queue of outgoing mail, and for accepting (or rejecting) incoming mail. Examples: sendmail, qmail, postfix, exim.

Mail User Agent (MUA): a program which provides a human user interface for reading and sending mail. Examples: elm, pine, mutt, Outlook, Netscape, Thunderbird.

Simple Mail Transport Protocol (SMTP): the protocol used between MTAs for sending mail from one host to another. This protocol is also sometimes used between an MUA and an MSA (see below). The official TCP port number for SMTP is 25.

Mail Submission Agent (MSA): a relatively new term in the e-mail field. This is the component of an MTA which accepts new mail messages from an MUA, using SMTP. (Traditional Unix MUAs send their mail using a pipe to one of the MTA's component programs on the same host. Most Windows MUAs use SMTP to talk to an MSA because there is no MTA on the Windows host.) Most MTA implementations use the same program as both their MSA and the part which accepts incoming mail from other hosts. In other cases, these functions are implemented separately. The official TCP port number for an MSA is 587 (although in many cases it's run on port 25).

Mail Delivery Agent (MDA): the component of an MTA which is responsible for the final delivery of a message to a local mailbox on disk. Sometimes this is a separate program, and sometimes it's built into the MTA.

Post Office Protocol (POP): a protocol used by some MUAs to retrieve mail from a user's mailbox on a remote server. Often written "POP3". The official TCP port number for POP3 is 110.

Internet Message Access Protocol (IMAP): a protocol used by some MUAs to retrieve mail from a user's mailbox on a remote server. This is a newer and more complicated protocol than POP, with a lot more functionality. The official TCP port number for IMAP is 143.

Email is based around the use of electronic mailboxes. When an email is sent, the message is routed from server to server, all the way to the recipient's email server. More precisely, the message is sent to the mail server tasked with transporting emails (called the MTA, for Mail Transport Agent) to the recipient's MTA. On the Internet, MTAs communicate with one another using the protocol SMTP, and so are logically called SMTP servers (or sometimes outgoing mail servers).

The recipient's MTA then delivers the email to the incoming mail server (called the MDA, for Mail Delivery Agent), which stores the email as it waits for the user to accept it. There are two main protocols used for retrieving email on an MDA:

• POP3 (Post Office Protocol), the older of the two, which is used for retrieving email and, in certain cases, leaving a copy of it on the server.
• IMAP (Internet Message Access Protocol), which is used for coordinating the status of emails (read, deleted, moved) across multiple email clients. With IMAP, a copy of every message is saved on the server, so that this synchronization task can be completed.

For this reason, incoming mail servers are called POP servers or IMAP servers, depending on which protocol is used.