|Part||What It Does||Examples|
|Mail User Agent (MUA)||The MUA is the program which the user uses to read and send e-mail. It reads incoming messages that have been delivered to the user's mailbox, and passes outgoing messages to an MTA for sending.||elm, pine, mutt|
|Mail Transfer Agent (MTA)||The MTA basicly acts as a "mail router". It accepts a message passed to it by either an MUA or another MTA, decides based upon the message header which delivery method it should use, and then passes the message to the appropriate MDA for that delivery method.||sendmail, postfix, qmail|
|Mail Delivery Agent (MDA)||The MDA accepts a piece of mail from an MTA and performs the actual delivery.||mail.local, procmail|
The MTA is the most important one of these. It is responsible for doing all the "intelligent" work of e-mail transfer. While it does not actually perform any of the delivery itself, it is the part which tells the other parts how to interact and what to do. In a sense, the MTA is the glue which holds the whole process together.
To illustrate how the three parts of the email system work together, here's a very general example. This is what happens when the user email@example.com sends e-mail to firstname.lastname@example.org: