20 This document is a specification of the basic protocol for Internet 21 electronic mail transport. It consolidates, updates, and clarifies 22 several previous documents, making all or parts of most of them 23 obsolete. It covers the SMTP extension mechanisms and best practices 24 for the contemporary Internet, but does not provide details about 25 particular extensions. Although SMTP was designed as a mail 26 transport and delivery protocol, this specification also contains 27 information that is important to its use as a "mail submission" 28 protocol for "split-UA" (User Agent) mail reading systems and mobile 29 environments. 56 58 Internet-Draft RFC5321-Numbered Dec 2008