2.2 What Does a Mail Transfer Agent (MTA) Do?

The Internet model of email delivery divides the process into several separate stages and the software into several parts. The two important kinds of software are the Mail User Agent (MUA) and the Mail Transfer Agent (MTA). The MUA is the program that permits a user to send and receive mail. Familiar mail programs such as Pine, Elm, and Gnus on Unix and Eudora, Pegasus, Outlook, and Netscape or Mozilla on PCs are all MUAs. Each MUA has a rather complex user interface, and has many features, such as composing and reading mail, moving mail among mailboxes, and selecting the order in which to read mail. But an MUA doesn't deliver mail to other users; for that it hands its messages to an MTA.

In the first stage of mail delivery, the message is submitted or injected to the MTA. Usually the message comes from an MUA, but it can just as well come from another program, such as a mailing list manager. The MTA examines the address(es) to which each message is sent, and either attempts to deliver the message locally if the address is local to the current host, or attempts to identify a host to which it can relay the message, relaying the message to that host. (If that last sentence sounds a little vague, it's deliberately so, because there are many different ways that mail relaying happens.) Each of these steps could fail a local address might not exist, it might exist but the MTA might be temporarily or permanently unable to deliver the message to it, the MTA might be temporarily or permanently unable to identify a relay host, or the MTA might be able to identify a relay host, but temporarily or permanently unable to relay messages to it. In case of permanent failure, the MTA sends a failure report back to the message's sender. In case of temporary failure, the MTA hangs on to the message and retries until either the delivery succeeds or eventually the MTA treats the failure as permanent.

Although the basic idea of an MTA is simple, the details can be complex, particularly the details of handling errors. Fortunately, qmail handles most of the details automatically, so administrators and users don't have to.



qmail
qmail
ISBN: 1565926285
EAN: 2147483647
Year: 2006
Pages: 152

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