This document is a specification of the basic protocol for Internet electronic mail transport. It (including text carried forward from RFC 5321) 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. The document also provides information about use of SMTP for other than strict mail transport and delivery. This document replaces RFC 5321, the earlier version with the same title, and supersedes RFCs 1846, 7504, and 7505, incorporating all the relevant information in them.