SMTP

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

SMTP is just one part of the TCP/IP protocol suite. SMTP standardizes communication between MTAs by defining how messages move from one computer's MTA to another computer's MTA, though it does not 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. The SMTP protocol is concerned only with moving messages and with this as a goal, it limits the formatting attributes of messages. SMTP transfers only ASCII text — no fonts, colors, graphics, or attachments — though as mentioned earlier, 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 that can communicate with an MTA.