What list?

Spam/Junk is a mathematical process based on the contents of the message against a locally generated database of text fragments based on your training over time (in my case decades) The learning algorithm uses those fragments as tokens to determine is text is SPAMMY or not, when a preset number is achieved the mail is considered junk/spam. It has nothing at all to do with the from address so there is no "SPAMMERS" address list.

So having instructed Thunderbird that an email received is Junk, and it will then automatically put future emails into the junk folder, how do I instruct it not do so?

So having instructed Thunderbird that an email received is Junk, and it will then automatically put future emails into the junk folder, how do I instruct it not do so?

I think that is something of an overreach, and an issue that support folk have battled with for years. Selecting an email as SPAM does not create some sort of blacklist/block listing on that address.

You decided a single email from a specific address was junk. Given that having done so and knowing that Thunderbird has no regard to the sender in determining a junk status (it does not appear to matter how many time I tell people they still think it matters. I already told you that yet you came back immediately with this idea that the sender is relevant again.) the worst outcome you might see is a slightly higher false positive rate for junk because of your choice has added HAM terms to the SPAM tokens. But in a database with years of other decisions in it the impact will be about zero. The learning part of the filter is known to need around 50 decisions about what is SPAM and another 50 on What is HAM to really start to learn anything because everything in the text is weighted statistically.

You can always reset the overall learning database to a newly installed state, but honestly I would not even consider doing so until I had seen many many false positives that telling Thunderbird were not junk did not fix over time. That is a simple as clicking the reset training data in the account settings. Details of that are included in the document I link below.

There are more details on the junk filtering here https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages just so I do not appear to be biased to only official documentation, Mozillazine put together an article on junk filtering back in 2004. They have updated it over the years. The last time in 2017. But it provides detailed background information that is as relevant now as it was in 2004, just many of the links and addons no longer exist.