Bingo! Your question wasn't silly at all. libsasl2-modules wasn't
installed and installing it cured the problem. post now works
beautifully. Many thanks, and apologies for this lacuna and the time
wasted.
Whew! What a relief! I really couldn't think of anything else that
might have caused this. So this was a combination of a legitimate bug
plus a packaging error.
Post-1.7 I had added some additional detail to -snoop that also included
the list of available client SASL mechanisms; if we had THAT code in the
distribution, it would have made figuring this out a lot easier. Which
reminds me that we should work on getting out a new release soon-ish.
--Ken