I just realized that the only thing different with my templates was that I send them BCC, so I did a test and sure enough, anything sent BCC from any account is erroring out. If I send a template using standard TO, then it goes just fine. So, it's the BCC that isn't working. Is this a known issue? All accounts are having this problem.

This is nothing to do with Thunderbird. That is why you have been entirely unsuccessful in any attempts to fix the issue yourself.

What you are encountering is spam filtering on your outgoing mail. The service is apparently provided by https://www.cloudmark.com based on the remote MTA used in your error message.

A quick google on the error AUP#SNDR indicates the message is determined to be spam. and I would guess the SNDR is some sort of abbreviation of sender. Fundamentally the spam filtering space is full of cowboys and as there are no rules and the customers are generally big all sorts of poor spam filters get into production environments.

One of the things that has come up over and over are those in the SPAM blocking industry blocking all mail that is addresses only BCC. Most folk that have had issues caused by that have simply added themselves to the email in the To:. Sometime with a dedicated address with the likes of gmail for the purpose

Well, the reason why I came here is because I've used Thunderbird for years with multiple accounts of different types and used BCC with no TO and have had zero issues with these exact same settings and providers. I did an update and then it stopped working immediately. So, 2 + 2 equals the update being the problem. Even if it's just something standard that changed or is being handled differently that is still a correct method, the point is something changed during the update to make BCC not work, changed the way the spammers are seeing the emails. I did test the email to myself right after I posted and it worked, so it's definitely the absence of a TO that they are now taking exception to. What doesn't make any sense at all is why it's been working great for all of these years and now, post update it won't work. That's super frustrating.

You can interpret it however you like. The issue is cloudmark reject mail that does not have more to the addressing that BCC.

It is interesting that when I google the issue it keeps coming back to folk who have their domain with bluehost, and I notice you have your domain with them as well. Things like https://forum.spamcop.net/topic/72574-suddenly-my-message-is-rejected-aupsndr/

You might want to plug in your domain name to this server and see what problems it identifies https://intodns.com/ Normally I would do it for you, but as you removed the relevant domain from the rejection message I can not.