I guess I'm of the opinion that such knowledge must be explicitly configured
if MTAs are going to fail if it is not accurate - MTAs should not be making
guesses.
Disallowing this sort of caching makes autodiscovery a losing strategy
in any environment which includes active attack as part of its threat
model. I suspect that we'll have to suck that up but we should do
so conscious of the tradeoff.
-Ekr