Definition Edit

SMTP (an acronym for Simple Mail Transfer Protocol):

specifies that a computer sending an e-mail must contain information identifying the sender, generally referred to as 'hello domain' information, as well as certain recipient information. The hello domain, in turn, contains the internet protocol ('IP') address and domain name of the sending party. IP addresses are unique strings of numbers; each computer accessing the Internet is generally assigned one. For convenience and efficiency, IP addresses of many computers are given textual equivalents known as 'domain names.' . . . Additional contact information is provided as well. . . .[1]
is a text-based client-server protocol where client (email sender) contacts the server (next-hop recipient) and issues a set of commands to tell the server about the message to be sent, then sending the message itself. The majority of these commands are ASCII text messages sent by the client and a resulting return code (and additional ASCII text) returned by the server.[2]

Overview Edit

SMTP was originally specified in 1982 as [RFC 821] and has undergone several revisions, the most current being [RFC 5321].

When e-mail is sent, the hello domain and sender information exist as part of the transmission information of the e-mail. E-mail servers and routers starting with the sender's Internet Service Provider ('ISP') relay the message forward until it reaches the e-mail servers of the recipient's ISP, which then direct the e-mail to the intended recipient. These server interactions include the presentation of the sending e-mail’s hello domain. Essentially, the sending computer identifies itself and presents the hello domain of the e-mail.

Each time the message passes through a different server, a record of that transaction is added to the routing information. Thus, using the e-mail's routing information, one can retrace the exact servers it went through to reach its destination, as if each post office or mail sorting facility added its own postmark to a letter. Furthermore, a server along this chain could choose to block the message from going further. The sender, however, has no way to control or predict the pathway his message will take to arrive at its destination. In fact, the message can be broken up and sent in pieces through multiple unique pathways. Despite this inherent unpredictability, the last server through which the message passes before reaching the recipient is his ISP's server. . . .[3]

The use of SMTP for email facilitates the sending of spam, since SMTP does not require an email message to contain accurate routing information, except for the intended recipient of the email. As a result, a spammer can “spoof” or falsify some portions or all of the header of an email message, making it virtually impossible to identify the true source of the email message.

SMTP uses port 25 for information exchange.


  1. Jaynes v. Commonwealth, 48 Va. App. 673, 680-81, 634 S.E.2d 357, 360-61 (2006) (full-text).
  2. NIST Special Publication 800-177, at 7.
  3. Id.

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