A footnote on the "find out from a database" issue. The more I
think about it, the more I conclude that the database lookup
(LDAP or otherwise) is the only rational way of implementing the
desired functionality here in a case that might involve relaying.
I'm reaching the same conclusion.
But, if database lookup is the only plausible implementation that
is consistent with the underlying design requirements, then we
should not be looking at capabilities negotiation in the SMTP
stream. Instead, we should be looking at a piece of protocol by
which the sending system (probably the MUA, rather than the MTA)
can figure out the capabilities of the recipient(s) and then
prepare the message, make address grouping decisions, etc.,
cleanly. It seems to me that would be more clear, would put the
responsibility back on the MUA or local system which has maximum
information about sender intent, would avoid layering violations
and other complications, and generally be a cleaner and more
efficient model.
I generally agree, or at least I think it's worth considering this
alternative. Which makes more sense:
- explicitly configuring every general-purpose SMTP server that acts
as a mail exchanger (and that supports conneg) to talk to a database
for each of the recipient domains that it serves?
or
- defining a standard recipient capability query mechanism
(say, something like rescap) and having a standard way to
advertise via DNS which servers to use to find capabilities
for recipients at a particular mail domain (say, something
like SRV records)
Problem is, you get different answers depending on whether:
- you build appliances that receive fax over the internet (in
which case it's simpler to just implement one server process
and you'd rather avoid imposing the burden of extra DNS setup
on your customers), or
- you build or operate general-purpose SMTP servers (in which
case you might rather avoid having to explicitly configure
those servers, and also avoid having your SMTP servers wait
on more database queries)
The two approaches are not mutually exclusive, but if we wanted
to support both query methods then it would be good idea to
understand how they interact sooner rather than later.
Keith