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.
As to how it came about, I can see that the Debian libsasl2-2 package "recommends" libsasl2-modules, but in default I use 'Apt::Install-Recommends "false"' on the grounds that in general "true" drags down just too much unwanted dead wood. I guess I will have to be more careful in future. Many thanks anyway. Tony -- Tony Stoneley Preferred email address a...@cam.ac.uk ...not withstanding anything any ISP may force into my "From:" field ---------------------------------------------------------------- Ken Hornstein wrote on Thu, 03 Feb 2022 14:29:30 -0500 I have one silly question: did you install the libsasl2-modules package? That's the package that actually contains the sasl mechanism for PLAIN (along with others). There should be something like "libplain.so" in a directory close to /usr/lib/x86_64-linux-gnu/sasl2 depending on your architecture.