Thunderbird learns by repeated checks of spam and non-spam. The more you work with it, the 'smarter' Thunderbird gets.

Not spam, scam. Spam detection can learn. Scam warnings are totally ignorant.

Scam detection is either on or off, with no other options, and I'm not aware of any documentation that explains the scam algorithm.

That is my point. There should be a way to mark a sender as safe instead of flagging the same newsletter every month.

That is my point. There should be a way to mark a sender as safe instead of flagging the same newsletter every month.

Sorry to be the naysayer, but no sender is safe when it comes to spam or scams. Most of the targeted scams, commonly refereed to as spear-phishing actually uses legitimate and known email addresses to fool the recipient. Have a look at this article https://www.digitalguardian.com/blog/what-is-spear-phishing-defining-and-differentiating-spear-phishing-and-phishing

As for the scam detection, it is very basic. It is documented in the support article here https://support.mozilla.org/en-US/kb/thunderbirds-scam-detection#w_thunderbirds-automatic-scam-filtering but if you want to go through the code it is located here https://searchfox.org/comm-central/source/mail/modules/PhishingDetector.jsm

Every month I get a newsletter from a known sender. It is the only thing I get from that sender. Every month it is marked as a potential scam. I am willing to take the risk for that sender and the email has passed all of my other security checks. So I'm left with two options:

   1.  Put up with the annoyance
   2. Turn off scam detection completely.

So far I have chosen #1. But I should not have to.

Every month I get a newsletter from a known sender. It is the only thing I get from that sender. Every month it is marked as a potential scam. I am willing to take the risk for that sender and the email has passed all of my other security checks. So I'm left with two options: 1. Put up with the annoyance 2. Turn off scam detection completely. So far I have chosen #1. But I should not have to.

Years ago I selected option 2. so you are preaching to the choir about the detection being more annoyance than help. But as this is a peer user support forum, not a feedback forum, the "should be features" are really off topic. Perhaps make your feelings known in the feedback forum here https://connect.mozilla.org if you wish to offer feedback or ideas for improvement.