What Is SMTP (Simple Mail Transfer Protocol)?

In Mercury is a MercuryE SMTP end-to-end client, a MercuryC SMTP Relaying client and an SMTP server MercuryS

This can be used for internal mail but your mail system.

SMTP is a relatively simple, text-based protocol: first, the sender of the message is specified, then one or more receivers and then the shipping information and content of the message. It is easy for an SMTP server to the test by using the telnet program. SMTP uses TCP port 25.

For the SMTP server for a domain to determine the MX record (MX = Mail eXchange) DNS uses.

History Of SMTP

SMTP began to be used worldwide in the early 1980s. Then it was a complement to UUCP. UUCP was appropriate to send e-mail between computers that are not always related. SMTP assumes that the receiver is always connected to the network.

Sendmail was one of the first (if not the first) mail transfer agent programs that use SMTP.

Since 2001, at least 50 different programs that support SMTP as a client (sender of messages) or server (receiving messages). Some popular SMTP server programs Postfix (IBM), qmail (from Daniel J. Bernstein), GroupWise (Novell) and Microsoft Exchange Server.

Because SMTP only as a pure ASCII text protocol was developed, it can not work with binary files (attachments). Standards such as MIME were developed to represent binary files so they can be sent over SMTP. Contemporary SMTP servers support the 8-bit MIME extension, as sending binary files almost as easy as regular text.

Send and Receive

In SMTP, the initiative always from the sender. This is called ‘pushing’ or ‘uploading’ said. There is desirable that the recipient is ready at any moment. Since private computers usually do not permanently connected to the Internet, is not suitable for an SMTP message to the ultimate destination to deliver. The message is requested by the provider of the recipient until the recipient connects to the Internet and retrieve the message. This clearance is for a protocol such as POP3 or IMAP, the initiative of the receiver off. This is called ‘pulling’ or ‘downloading’.

Spam and Security of SMTP

One of the limitations of the original SMTP is that there is no possibility to verify the sender really is who he claims to be. This limitation is exploited to send spam, with a fictitious sender of an e-mail can be specified, so spam is still a major problem. So most ISPs block port 25 (the SMTP server port) for customers. SMTP thoroughly changed or replaced by a completely new protocol is not considered because it is so widely disseminated and implemented.

To present to verify that the sender really is the person who he claims to be the SMTP AUTH extension developed.

~Source: Wikipedia.