9.1. Background: MTA and SMTP Security

MTAs move email from one host or network to another. This task contrasts with that of Mail Delivery Agents (MDAs), which move mail within a system (i.e., from an MTA to a local user's mailbox, or from a mailbox to a file or directory). In other words, MTAs are like the mail trucks (and airplanes, trains, etc.) that move mail between post offices; MDAs are like the letter carriers who distribute the mail to their destination mailboxes. Procmail is one popular MDA on Linux systems.

In addition to MTAs and MDAs, there are various kinds of email readers, including POP3 and IMAP clients, for retrieving email from remote mailboxes. These clients are also known as Mail User Agents (MUAs), of which Mutt, MS-Outlook, Pine, and Evolution are popular examples. There is no real-world analogue of these, unless your letters are handed to you each day by a servant whose sole duty is to check your mailbox now and then. But we're not concerned with MUAs or MDAs, except to mention how they relate to MTAs.