1) Is your Thunderbird set to only ask for a password within X minutes? If so was this attempt for second email within said time frame?

2) Were the email passwords the same? ( bad habit but it's more common that I'd like to admit seeing in the wild.

3) How were passwords changed initially ? via a webui or within Thunderbird?

4) have you tried cp -r ~/Library/Thunderbird/Profiles/xxxxxxxx.default/ ~/Documents/Thunderbird and tried with a fresh instance to rule out a new config mismatch or plugin / theme /etc ?

Please find attached a screenshot of an empty error console with a look at where it is, given the reported errors yours should be loaded with content please sanitize any personal / confidential info and share the sanitized screengrab to also help troubleshooting efforts.

Get to that via the 'hamburger' (three horizontal lines) icon > Tools > Error Console.

Got the screen shot. Working on the rest.

Those first few ( up top) allude to an extension consider launching tbird in 'safe mode' and see if errors or issues persist. Also that very last one alludes to a local dns issue consider checking your discoveryd logs (dns resolver) and / or flushing then with:

sudo discoveryutil mdnsflushcache

Thunderbird has decided to ask me for the outgoing password. Don't know why.

In my haste I forgot to check the 'Remember Password' box, so the password does not appear in 'Saved Passwords,' but it keeps sending subsequent emails anyway. Hopefully it will ask again if it needs to.

Thank you very much.