Re: The job of an MSA

> [email protected] wrote:

> > First, RFC 2746 is quite clear: Port 25 MAY be used for SUBMIT if for whatever
> > reason 587 cannot be used.

> 2476 -- otherwise OK.

> > More generally, what port something is run on is largely irrelevant; the
> > important issues are service and protocol semantics.

> If the protocols are to be treated as fundamentally different -- and 2476
> says that an MSA runs ESMTP with some restrictions (N.B. not extensions) --
> then that raises the question of how a client which connects to a server
> on port 25 is able to determine whether that server is an MSA or an MTA.

It would have to be configured to know. But this is for the most part an
entirely academic point, since most clients are only prepared to perform
submission activities. Clients prepared to do relaying, with all it implies,
are hardly the norm.

> > Of course this doesn't mean someone won't attempt to use SUBMIT-specific
> > extensions in regular SMTP.

> At the moment, as I understand it, there are no such "extensions", there
> are only "restrictions" [RFC 2476 section 3.1].

> > But just because some clueless ISP engages in
> > egregiously bad practice is no reason to break our protocol specifications. I
> > am dubious about the value of such an extension in any case for all the reason
> > I've previsouly given; I would vehemently oppose making it part of regular
> > SMTP.

> It may be the case that presence of one or more hypothetical submission-
> specific extensions might serve to distinguish an MSA from an MTA. That
> would mean that the presence of such an extension would indicate that the
> server in question is an MSA (running ESMTP), not an MTA (running ESMTP).

> Of course if a particular MSA doesn't support any of the hypothetical
> MSA-specific extensions, there is still the issue of how a client is
> supposed to be able to determine whether it has connected to an MTA or
> an MSA.  What do you suggest?

The idea of offering a specific extension saying "this is a submission server"
or "this is a SMTP relay server" was considered and rejected.  I doubt very
much if revisiting this issue at this time will produce a different result. As 
such, should the issue of a client needing to know the different ever come up,
the only way to do it would be with a configuration option.

				Ned