The Glossary referred to Mr. KANBE, Takahiro's mail as it is. Thank you, Mr. KANBE! And this page is translated by TAKIGUCHI, Masao <masao@cadence.com>. Thank you, Mr. TAKIGUCHI.
An entity which delivers mail messages among systems. Some of them cooperate to compose MTS (Message Transfer System).
An entity which sends and receives mail messages with reflecting on the intension of users (human beings or programs) of mail system.
The distinction between MUA and MTA is only made from model point of view. And they can not be distinguished strictly in their actual implementation. For example, sendmail takes charge of greater part of MTA, but at the same time it also does some procedures like MUA.
It is sent as the mail delivered from MUA to MTA or from MTA to MUA and it actually consists of the part of envelope and contents.
A part of message which has information of sender and recipient. Fundamentally, it is information used by MTA for delivery and it is different from main body of the mail (contents) which MUA sends. It is invisible or can be seen partly in the following headers as its traces:
It is possible that names of sender and recipient of envelope are renamed to others by the procedure of alias or mailing-list.
(It is information exchanged by MAIL/RCPT command in SMTP and exchanged by protocol in X.400.)
The main body of the mail delivered from MUA to MTA or from MTA to MUA. It is usually called "message of the mail".
In SMTP, it consists of header and body which are delimited by the first empty lines. And it based on that RFC822 defines.
(It is content based on memorandum in SMTP and MIME enables structurized content to be treated. X.400 allows structurized contents to be deal with by nature.)
A part of contents which includes additional information used by MUA and so on:
It is possible that information about sender and recipient specified in `From:', `Sender:', `To:', `Cc:', `Bcc:' and other headers doesn't correspond with that of envelope.
Neither sender nor recipient in the header are basically rewritten, except at posting time and except in the case that there are some policies to be rewritten.
SMTP doesn't distinguish clearly delivery from MUA to first MTA. And following matters should be executed in the stage of posting.
Unless special mail gateway is used, these information should not be rewritten except in the stage of posting.
In the stage from MTA to final MTA, procedure for headers is restricted as follows:
A case where address of their own organization is positively rewritten for some reason can be regarded as one kind of extension of mail gateway.
A stage where mail is stored in user's mailbox. And in SMTP, `Return-Path:' header which specifies sender of envelope is added. It is not right procedure to add `Return-Path:' header in the stage which is not final delivery.
A part of contents equivalent to text which user sends.
A line descibed by mail.local(/bin/mail) as delimiter of messages. It is not body of message. (It doesn't follow any rule like RFC822, either.)
Because it is defined to start "From ", there are some case that procedure for a line in the body starting with same pattern causes problems.
In the world like sendmail, delivery of mail ends in the stage where the processing have terminated normally.