In the TCP/IP protocol suite, electronic mail in a format specified in RFC-822 [SMTP:2] is transmitted using the Simple Mail Transfer Protocol (SMTP) defined in RFC-821 [SMTP:1].
While SMTP has remained unchanged over the years, the Internet community has made several changes in the way SMTP is used. In particular, the conversion to the Domain Name System (DNS) has caused changes in address formats and in mail routing. In this section, we assume familiarity with the concepts and terminology of the DNS, whose requirements are given in Section 6.1.
RFC-822 specifies the Internet standard format for electronic mail messages. RFC-822 supercedes an older standard, RFC-733, that may still be in use in a few places, although it is obsolete. The two formats are sometimes referred to simply by number ("822" and "733").
RFC-822 is used in some non-Internet mail environments with different mail transfer protocols than SMTP, and SMTP has also been adapted for use in some non-Internet environments. Note that this document presents the rules for the use of SMTP and RFC-822 for the Internet environment only; other mail environments that use these protocols may be expected to have their own rules.