[TriLUG] new "feature" in Thunderbird on Ubuntu 18.04

[Pete Soper via TriLUG]

Starting a few weeks ago I stopped being able to send myself email
messages with Thunderbird 60.2 on a 64 bit Ubuntu 18.04 system that has
regular updates applied unconditionally (i.e. whatever the standard
Ubuntu update system uses to periodically get permission to update I
tell it "yes, I'm willing to deal with subtle regressions that mght
affect my mental health.")

When "send" is hit for a message to my main email address a dialog box
comes up to say "Sending of the message failed", another dialog comes up
labeled "Sending message" with a green progress line endlessly
circulating from left to right until I click "OK" on the first dialog.
Nothing shows up in /var/log/syslog, nor /tmp. The size of /var/log
doesn't change even after a dozen failed attempts (figuring it would
grow by a block with enough error messages in a log file someplace).
I've also piped find . -f file into ls before/after to find the files
that are new or changed in size under the Thunderbird ".default" file
but can't find any clues there.

Email messages to all addresses except the one that I'm using within
Thunderbird are unaffected.

Could somebody help me find the bread crumbs to get a clue about what's
happening? Is there some kind of logging mode I could enable with SMTP?

I haven't tried an older Thunderbird version yet.

Thanks,
Pete

(Why would I need to send myself email? I have a note system using email
filters keyed off character sequences in the subject line of messages
and this has hosed that tool.)