Why sendmail Is So Complex?

In its simplest role, that of transporting mail from a user on one machine to another user on the same machine, sendmail is almost trivial. All vendors supply a sendmail (and a configuration file) that will accomplish this. But as your needs increase, the job of sendmail becomes more complicated, and its configuration file becomes more complex. On hosts that are connected to the Internet, for example, sendmail should use the Domain Name System (DNS) to translate hostnames into network addresses. Machines with UUCP connections, on the other hand, need to have sendmail run the uux program.

The sendmail program needs to transport mail between a wide variety of machines. Consequently, its configuration file is designed to be very flexible. This concept allows a single binary to be distributed to many machines, where the configuration file can be customized to suit particular needs. This configurability contributes to making sendmail complex.

When mail needs to be delivered to a particular user, for example, the sendmail program decides on the appropriate delivery method based on its configuration file. The decision process might include the following steps:

·         If the recipient receives mail on the same machine as the sender, sendmail delivers the mail using the /usr/sbin/mail.local program.

·         If the recipient’s machine is connected to the sending machine using UUCP, it uses uux to send the mail message.

·         If the recipient’s machine is on the Internet, the sending machine transports the mail using SMTP.

·         Otherwise, the mail message might need to be transported over another network (such as Bitnet) or possibly rejected.

[ Read More ]

Defination of MUA, MTA & MSA (sendmail)

MUA

A mail user agent (MUA) is any of the many programs that users run to read, reply to, compose, and dispose of email.

Examples of MUAs also exist for PCs. Eudora and Claris-Works are two standalone MUAs. Netscape and Explorer are web browsers that canalso act as MUAs. Thunderbird is an open source MUA from the folks at Mozilla. Many MUAs can exist on a single machine. MUAs sometimes perform limited mail transport, but this is usually a very complex task for which they are not suited.

MTA

A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office does. Usually, there is only one MTA on a machine. The sendmail program is an MTA.

MSA

sendmail also recognizes the role of a mail submission agent (MSA), as defined in RFC2476. MTAs are not supposed to alter an email’s text, except to add Received:, Return-Path:, and other required headers. Email submitted by an MUA might require more modification than is legal for an MTA to perform, so the new role of an MSA was created. An MSA accepts messages from an MUA, and has the legal right to heavily add to, subtract from, and screen or alter all such email. An MSA, for example, can ensure that all hostnames are fully qualified, and that headers, such as Date:, are always included.

[ Read More ]

Mail Server Basic

Imagine yourself withpen and paper, writing a letter to a friend far away. You finish the letter and sign it, reflect on what you’ve written, then tuck the letter into an envelope. You put your friend’s address on the front, your return address in the lefthand corner, and a stamp in the righthand corner, and the letter is ready for mailing. Electronic mail (email for short) is prepared in much the same way, but a computer is used instead of pen and paper.

The post office transports real letters in real envelopes, whereas sendmail transports electronic letters in electronic envelopes. If your friend (the recipient) is in the same neighborhood (on the same machine), only a single post office (sendmail running locally) is involved. If your friend is in a distant location, the mail message will be forwarded from the local post office (sendmail running locally) to a distant one (sendmail running remotely) for delivery. Although sendmail is similar to a post office in many ways, it is superior in others:

·         Delivery typically takes seconds rather than days.

·         Address changes (forwarding) take effect immediately, and mail can be forwarded anywhere in the world.

·         Host addresses are looked up dynamically. Therefore, machines can be moved or renamed and email delivery will still succeed.

·         Mail can be delivered through programs that access other networks (such as Unix to Unix Communication Protocol [UUCP] and Bitnet). This would be like the post office using United Parcel Service to deliver an overnight letter.

[ Read More ]