SMTP service
Click on the red underlined text to get to the source
... This memo uses the mechanism defined in [5] to define extensions to
the SMTP service whereby a client ("sender-SMTP ...
... service extension is therefore defined:
(1) the name of the SMTP service extension is "Message Size
Declaration";
...
... Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extensions", RFC 1651(-> 1869(-> 2821prop)), MCI, Innosoft, Dover Beach Consulting, Inc., Network Management Associates, Inc., Silicon Graphics ...