SMTP Service Extensions

SMTP provides a reliable and efficient delivery mechanism for sending e-mail messages. Although its role has been evident for more than 10 of years, it is essential to expand its function. Extensions to the SMTP service we'll cover both the envelopes and the content in the mail forwarded by SMTP. We write letters and envelopes and Chimpi, and we can borrow letters from our lives to help us understand.

(1) The SMTP envelope is relatively easy to understand and is delivered as a series of SMTP protocol units, which includes the sender address, the delivery mode, and one or more recipient addresses. If you are not sure, see the SMTP protocol standard.

(2) As for the content, it is composed of two parts, part of the letterhead, part of the letter, letterhead is a domain/value pairs (a domain, a value), if the letter structure, its structure is in MIME construction. Content is essentially text, and is generally composed of ASCII code, but because of the use of mime, so this limit should also be no, but the letterhead is not, generally should use ASCII notation. Although the SMTP protocol is a good protocol, but the extension of it is unavoidable, this article mainly describes an extension method, using this extension method, the server and users can know each other using the extension, how much use, if the communication.

Here we want to let you know one of the experiences in network protocols: The more parameters, the faster the death, the less the argument, the longer it lasts. Too many parameters, not conducive to use, can not be promoted, sooner or later will be replaced by other agreements. This is also in line with the basic principles of science, simple. This means that the implementation must be careful, if not careful will get far less than pay, sometimes can not provide any benefits.

Let's take a look at the EHLO command, where customers who support the SMTP service extensions should start the SMTP session with the EHLO command, rather than the usual helo command. If the server also supports it, return a confirmation response and return the failure response if not supported. Because the EHLO command is introduced, the first command to start the session can be helo or EHLO.

As a result, new parameters are introduced, so the length of the Mailfrom and Rcptto commands for SMTP is also 512 bytes, but the lengths to introduce new parameters must be described so that the implementation can prepare buffers.