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.

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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.

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Difference between POP3 and IMAP ?

POP3 and IMAP

"On our system, EVERY e-mail account acts as Both a POP3 and an IMAP account. Really they are just two different ways to read the mail on your account with us"

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• With POP3, the mail is "popped" off our server and onto your home computer. There's no synchronizing going on, just copying of mail files from one place to another. In most POP clients, you can choose to leave a copy of the mail on the server, or to delete it from the server when you download it. POPPING is good if you just want to get online, download your email, and get off line again.
• With IMAP your email client is constantly connected to our server, and doesn't make local copies of your messages. IMAP is great if you access your email from lots of different machines, since it keeps all the messages in one place. It's also good if you have a slow connection, because it only downloads the email headers until you choose to read a whole message. That's great for deleting spam by just reading the subject line! IMAP needs a constant connection to the Internet the whole time you're doing your email.

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