SMTP

As important as the format of electronic messages is the transfer protocol for which that format was created. This is SMTP, the Internet protocol for electronic mail. It is defined in RFC 821, the companion document to Summary of RFC 822.

SMTP is just one part of the TCP/IP protocol suite. SMTP standardizes communication between message transfer agents (MTAs) by defining how messages move from one computer's MTA to another computer's MTA. SMTP does not, however, specify the message's path to the destination computer. This path can be direct, or it can lead through intermediate computers in a process known as store and forward.

Because the SMTP protocol is concerned only with moving messages, it limits the formatting attributes of messages. SMTP transfers only ASCII text, with no fonts, colors, graphics, or attachments. Multipurpose Internet Mail Extensions (MIME) extensions provide workarounds for these limitations.

In the early days of the Internet, SMTP gained widespread use through its close connection to UNIX-based systems. At first it was used only with UNIX tools, such as the e-mail program sendmail. Today SMTP is the still the standard e-mail protocol for the Internet, but Internet hosts use a wide variety of operating systems, and people can read and send their mail with any user agent (UA) that can communicate with an MTA.