http://www.rfc-editor.org/rfc/rfc2595.txt
RFC 2595: Using TLS with IMAP, POP3 and ACAP, C. Newman1999-06-01T23:00:00-00:00Recognizing that such sites will desire simple password authentication in combination with TLS encryption, this specification defines the PLAIN SASL mechanism for use with protocols which lack a simple password authentication command such as ACAP and SMTP. [STANDARDS-TRACK]