D. J. Bernstein
Internet mail
SMTP: Simple Mail Transfer Protocol

The HELP verb

A HELP request asks for information from the server. It may include a parameter. The meaning of the parameter is defined by the server.

The server must use code 211 or 214 if it accepts the request, 421, 500, 501, 502, or 504 if it rejects the request:

     220 heaven.af.mil ESMTP
     214-SMTP servers help those who help themselves.
     214 Go read https://cr.yp.to/smtp.html.
     504 I don't support HELP parameters.

The server's envelope is unchanged by HELP.

There is a HELP extension that promises support for the HELP verb. Many servers support HELP but don't announce the HELP extension. Clients don't pay attention to the HELP extension; they simply try the HELP request.