It seems like the best time for this to happen would be during submission
where the mua is in a much better position to deal with the failure than
some other mta in the middle. How evil would it be for the msa to remember
which hosts it's talked to recently that accept a given extension to
propagate back to its clients?
Not at all, assuming a reasonable version of "recent." I would guess
something on the order of a day would be recent, but you surely have or
could collect data that can tell you how often the extensions offered by
other MTAs changes to figure out a good TTL.
I don't know how this scales, but I think some MSAs also do call-forwards,
connect to the next server during the submission session to see if it
handles the required extensions.
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
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