Regarding caching: I see no problem with MTAs that obtain conneg strings
from a database caching those lookups. OTOH I think caching is extremely
dubious as a mechanism for propagating conneg strings in the direction
from the recipient's MTA to the sender's UA, at the rate of one hop per
email message transmitted.
The bottom line is that we're dealing with partial solutions in every case
here. This makes it easy to criticize any given scheme. IMO this also makes it
very much the WG's call as to how to proceed.
I think this is quite a reasonable statement if we are talking about a
fax-specific extension; less reasonable if we are talking about a generic
SMTP extension.
And regardless, I think more discussion is needed in the document about
which parties have the responsibility for conversion (presumably the
sender's UA), and which parties have responsibility for providing
recipient information (presumably *all* of the listed MXes for that domain) -
this in addition to addressing several other issues that have been raised.
Keith