SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy to test a SMTP server using the telnet program. SMTP uses TCP port 25. To determine the SMTP server for a given destination, the MX (Mail eXchange) DNS record is used.
| Application layer | FTP | SMTP | HTTP | ... |
| Transport layer | TCP | UDP | ||
| Network layer | IP ICMP | ARP | ||
| data link layer | Ethernet | Token Ring | FDDI | ... |
SMTP started becoming widely used in the early 1980s. At the time, it was a complement to UUCP which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.
Sendmail was one of the first (if not the first) mail transfer agent to implement SMTP. As of 2001 there are at least 50 programs that implement SMTP as a client (sender of messages) or a server (receiver of messages). Some other popular SMTP server programs include IBM's Postfix, D. J. Bernstein's Qmail, and Microsoft Exchange.
Since this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as MIME were developed to encode binary files for transfer through SMTP. Today, most SMTP servers support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.
SMTP does not allow one to retrieve messages from a remote server. To do this one must use POP3 or IMAP.
One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined.
In spite of this, E-mail spamming is still a major problem. Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP.
For this reason, there are a number of proposals for sideband protocols that will assist SMTP operation. The Anti-Spam Research Group of the IRTF is working on a number of proposals for providing simple source authentication that is flexible, lightweight, and scalable. The most likely proposal to be accepted is the Sender Permitted From protocol.SMTP security and spamming