This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments. This document replaces the earlier version with the same title in RFC 5321. [[CREF1: Note in Draft: Except for the last sentence, the above is unchanged from 5321 and may need adjusting in the light of RFC 6409 as an Internet Standard.]]