Eddy Younger (email@example.com)
Software Components of the Email System
There are principally three classes of software components involved in transfering a mail message from the sender to the recipient:
- MUA – mail user agent, used to read, compose and post mail.
- MTA – mail transport agent. MTA’s at the source and destination hosts (and possibly also intermediate hosts) pass the messages from one to another
- MDA – mail delivery agent. At the ultimate destination host, the MDA receives the message from the local MTA and delivers it to its ultimate destination, usually the recipient’s mailbox file.
In the modern world almost all email transport is achieved using SMTP – the Simple Mail Transport Protocol – or its Extended variant ESMTP. MTA’s speak to each other in (E)SMTP. You can send mail without using a MUA if you wanted to, by talking SMTP directly to the MTA, and it used to be possible to do all kinds of nefarious things by doing so, though thankfully most mail servers are much more secure in these days of Skript Kiddies and Spam.