The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol in widespread use today. A message sent using SMTP consists of three components: the message envelope, the message header, and the message body. The message envelope contains the information necessary to deliver the message to the recipient. This includes the email address of the recipient(s), as well as any other routing information that may be required. The email address is usually in the format "UserName@DomainName". The message header contains information about the sender of the message, the subject of the message, and when the message was sent. The message body is the actual content of the email.