Thunderbird does not have a blacklist; the email that is spam today may not be tomorrow. There does exist a spamassassin feature and you should tick that. this URL may assist. https://blog.thunderbird.net/2024/09/thunderbird-and-spam/

I have always had Spam Assassin checked in all of my accounts. Usually, when I right-click a message that I think is spam and select Mark | As Spam, the message goes to my Spam folder, and I will never see another message from that email address in my in-box again. That behavior has changed recently. I did look for an entry in Tools | Message Filters, but I only saw rules that I manually created there. They work fine. If you actually read the URL that you noted above, it pretty much says that Thunderbird learns from your actions. Something is broken.

No Thunderbird's behavior has not changed. You have observed something you perceive as changed. That does not make it a change.

Here is the source code for the junk filtering https://searchfox.org/comm-central/source/mailnews/extensions/bayesian-spam-filter/nsBayesianFilter.cpp and the header file is https://searchfox.org/comm-central/source/mailnews/extensions/bayesian-spam-filter/nsBayesianFilter.h

You will find each change marked in the margin with the date and bug that implemented the change. I am not seeing anything there for years.

What has changed in December or even recently? My motivation is about nil on this topic so you have a look.

If you actually read the URL that you noted above, it pretty much says that Thunderbird learns from your actions.

It does not say the filter has any regard to the sender, and I can assure you it does not and never has. You assume learning has something to do with the message sender and it does not.

I do not use SpamAssassin, or I should say my provider does not and it is something that the mail provider supports and runs on the mail server. Thunderbirds only role in that is to read the header SpamAssassin sets and honor its decision. Perhaps SpamAssassin is no longer treating those email as spam so Thunderbird is not either or perhaps your mail provider does not actually support SpamAssassin. Thunderbird will let you set the filter even if it never sees a header to act on. I can not really comment if you provider uses SpamAssassin as you make no mention of who that is. But Microsoft, Google Yahoo and GMX do not use SpamAssassin. So the majority of folk to not have it offering guidance, regardless of their preferences in Thunderbird what mail provider are your accounts hosted with?

Thunderbird does not use a blacklist, which is reason why blog post nor the support document on which it is based make no reference to one https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages and https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages To quote those sources on how to block individual senders

Other Ways to Block Unwanted Messages Thunderbird’s adaptive junk filter is not an absolute barrier against messages from specific addresses or types of messages. You can use stronger mechanisms to block unwanted messages: Create Filters Manually You can manually: Block some senders. Create filters to block messages based on subject or other criteria.

Thank you. John