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Definitions of some terms related to email and email servers, like MUA, MTA, MDA, pop server, pop3 protocol, imap server, imap protocol, smtp protocol, etc.

See:

Contents

Introduction

In the RFCs which specify how email works on the Internet, certain terminology has been defined, including the following. (These definitions are in my own words, without looking them up -- see BLT.)

MUA (Mail User Agent)

If you come from Windows, you can think of this as your email client -- the software that you use to read and write email. In Windows the software can talk directly to an ISP's email server using smtp and pop3 or the imap protocol.

Some examples of an MUA include: Eudora, KMail, Microsoft Outlook, Sylpheed, Netscape Mail (the email client in Netscape Navigator), Elm, Mutt, and Pine.

Elm, Mutt, and Pine are Linux MUAs which, AFAIK, do not have the capability of talking directly to an ISP's server using smtp and pop3 or the imap protocol, but instead pick up and deliver mail to specified directories (or files) on the local machine. An MTA takes the mail from the "outbox" directory, and an MDA receives mail from an MTA and delivers it to "inbox" directories.

Hopefully the sketches on EmailServerSketches make this more clear, and clarify some alternate possibilities.

MTA (Mail Transport Agent)

A program that takes mail from a specified directory and transports it to other email servers for relaying to it's final destination. See EmailServerSketches. (This is oversimplified and possibly not exactly correct -- this is my impression at this point it time.)

MDA (Mail Delivery Agent)

A program that accepts mail from an MTA and distributes to the recipients on a local machine. (This is oversimplified and possibly not exactly correct -- this is my impression at this point it time.)

When mail must be delivered to (or read from) other machines on a LAN, a pop (or imap) server is required.

pop server

A program that allows a user to download mail from a user's mail directory to another machine, using the #pop3_protocol. A pop3 server is often included with an imap server.

In a Windows style email client, a pop server is necessary on the ISP's machine to allow the email client to download (incoming) mail from the ISP.

imap server

A program that allows a user to read and write mail in his directory from another machine, without downloading the mail to that other machine. It uses the #imap_protocol.

In a Windows style email client, an imap server is necessary on the ISP's machine to allow the email client to view incoming mail from the ISP without downloading it, and to create outgoing mail on the ISP's machine and send it from there.

pop3 protocol

See RFC <later>. A protocol for

See #pop_server.

smtp protocol

See RFC <later>. A protocol for

imap protocol

See RFC <later>. A protocol for

See #imap_server.

fetchmail

Or getmail -- I don't know that there is a generic name for this type of program: When a Linux workstation using an MTA is not connected to the Internet full time (i.e., a dial up connection), a program like fetchmail is used to download mail from the ISP's server on demand. This can be done manually, via a cron job, or via a program that detects when the connection is up (if_up).

Fetchmail can be used without an MTA for similar purposes, like if Procmail (an MDA) is used without an MTA. In the standard configuration of fetchmail, it downloads mail via the pop3 protocol and presents it to port 25 where the MTA receives it.


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  • RandyKramer - 03 Apr 2002
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