Книга: Fedora™ Unleashed, 2008 edition
The Mail Delivery Agent
The Mail Delivery Agent
SMTP is a server-to-server protocol that was designed to deliver mail to systems that are always connected to the Internet. Dialup systems connect only at the user's command; they connect for specific operations, and are frequently disconnected. To accommodate this difference, many mail systems also include a mail delivery agent, or MDA. The MDA transfers mail to systems without permanent Internet connections. An MDA is similar to an MTA (see the following note), but does not handle deliveries between systems and does not provide an interface to the user.
NOTE
Procmail and Spamassassin are examples of MTAs; both provide filtering services to the MTA while they store messages locally and then make them available to the MUA or email client for reading by the user.
The MDA uses the POP3 or IMAP protocols for this process. In a manner similar to a post office box at the post office, POP3 and IMAP implement a "store and forward" process that alleviates the need to maintain a local mail server if all you want to do is read your mail. For example, dialup Internet users can intermittently connect to their ISPs' mail servers to retrieve mail by using Fetchmail — the MDA provided by Fedora (see the section "Using Fetchmail to Retrieve Mail," later in this chapter).
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