This document reflects apparent ambiguities in RFC 821 that remain after the clarifications in RFC 1123. It also includes some material from RFC 1123 that appeared to need amplification. These have been identified in multiple ways, mostly by tracking flaming on the header-people list and problems of unusual readings or interpretations that have turned up as the SMTP extensions have been deployed. It is important to note that everything here is in response to some identified confusion or bad behavior, not just paranoia. 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 posting' protocol, as recommended for POP3 [RFC-POP3] and IMAP4 [RFC-IMAP4]. Except when the historical terminology is necessary for clarity, this document uses the current 'client' and 'server' terminology to identify the sending and receiving SMTP processes, respectively. A companion document discusses mail bodies and formats: RFC-822, MIME, and their relationship.