On Thu, Jun 28, 2018 at 2:00 PM smallglt <gtackett@yahoo.com> wrote:
I'm posting /this/ update because one subscriber to this list has informed
me that he received notification of that update /in his spam folder/, with
some kind of message about my email address not passing validation by Yahoo
Mail (my email provider).

I've seen this problem on other lists with some (not all) yahoo accounts. Your last messages did get flagged as spam on my Gmail. The reason: 

This message has a from address in yahoo.com but has failed yahoo.com's required tests for authentication.  

My understanding is that to avoid this problem, you must use Yahoo.com's outgoing SMTP server for sending messages. When I looked at the SMTP headers of your message, it appears to have gone through nabble.com from your localhost. Spammers often spoof the "from" address (technically you're doing the same thing if your email isn't at nabble.com), so it looks suspicious. 

I suggest taking other questions off list (email me directly) if you need more info. Hope it helps.