On Thu, 29 Nov 2007 13:38:50 EST, Hector Santos said:
For example, a command such as NOOP, whose successful execution does
not offer the SMTP client any new information, will return a 250
reply. The reply is 502 when the command requests an unimplemented
non-site-specific action. A refinement of that is the 504 reply for
a command that is implemented, but that requests an unimplemented
parameter.
Note that a *valid* 421 due to a server shutdown or similar incident *does*
offer the SMTP client new information - the fact that a shutdown or temporary
issue has commenced. So the "will return a 250" only matters if the
server *doesn't* have anything new to say - but it's totally free to signal
a new condition via a 421 or similar.
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