What is the SMTP 'NOOP' command used for?

Question

What is the purpose of the NOOP in the SMTP spec?

It says:

   This command does not affect any parameters or previously entered
   commands.  It specifies no action other than that the receiver send a
   "250 OK" reply.

   This command has no effect on the reverse-path buffer, the forward-
   path buffer, or the mail data buffer, and it may be issued at any
   time.  If a parameter string is specified, servers SHOULD ignore it.

   Syntax:

      noop = "NOOP" [ SP String ] CRLF

Answer 1

A quick google check, turned this up (source):

The simplest of these commands is NOOP, useful mostly in testing to avoid timeouts; it does nothing and can generate only a successful response, with no change in state...

Answer 2

On IIS 5.0, 7.5 and possibly other versions, sending too many NOOPs will get you in trouble as I found out recently. IIS will return this: "421 Too many errors on this connection---closing" and close the connection on you.

Here is a reference to someone who encountered this same problem...

Here is a cryptic reference to this on microsoft's support site here.

NOOP CRLF

The SMTP-receiver should send a 250 reply to the NO OPERATION command. The only other possible responses are a 500 reply, indicating incorrect command syntax, or the 421 reply, indicating that the SMTP service will be shutting down shortly.