Well, this is interesting. Based on your reply, I stopped trying to figure out any difference between the admin messages and the posts. Instead I removed the settings which routed the outgoing messages through MailerSend. That worked. I received both a welcome message and a post from a list I created just for this test. Then I restored the settings, and it still worked! Without the external routing, my messages ended up in spam but at least they were delivered. Routed through MailerSend they get delivered to my inbox.
I'll do some more testing because this all seems strange and I'm not convinced it's fixed.
BTW the errors that I had been getting were, at first: response exception: (450, b'The from.email must be verified. The from.email must be verified.') but later changed to: response exception: (450, b'Sorry, please try again. 429 Too many requests.')
I had assumed both errors were being generated by MailerSend but now I'm not sure
I'm not sure it's fixed but if not, I'm certainly closer. Thank you