http://www.rfc-editor.org/rfc/rfc1869.txt
RFC 1869: SMTP Service Extensions, J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker1995-11-01T23:00:00-00:00This memo defines a framework for extending the SMTP service by defining a means whereby a server SMTP can inform a client SMTP as to the service extensions it supports. [STANDARDS-TRACK]