I take it you are a native English speaker? I only ask because Thunderbird uses specific terminology and I'm not sure if the subtleties are preserved when it's translated into other languages.

Thunderbird doesn't use the word "Spam" not does it operate a "Spam" folder. In its English language version, it uses the words "Junk" and "Scam" for suspicious messages.

All the signs here are that your ISP or email provider are running some security software that marks some messages as Spam and handles them accordingly.

You can filter until you're blue in the face, but if the message has been categorised as "Spam" and moved to a different folder before it gets to Thunderbird, then the damage has already been done. In my own case, at present all my messages from this site and the associated newsgroup are being treated as spam by googlemail, and it seems there isn't anything I can do about it. All the relevant addresses are in the gmail account's address book, which ought to do the trick. But no…

My mistake...I used the word spam when I should have used junk. My email provider is Yahoo, so I went to the Yahoo Mail site and sure enough, it is marking them as spam. I have adjusted that setting, and will let you know if that does the trick, and keeps TB from marking them as junk. Thanks for the response!

Your problem is Yahoo.

Yahoo decide mail is spam and nothing I have ever tried changes that. Most spam tools scan your mail on arrival so once you have done your manual change it stays changed. Yahoo appear to constantly scan all folder and just change not spam mail back to spam. They also offer no way to opt out of their spam filtering. Apparently they know best.

I have tried rules on the yahoo web site. I have tried filters in Thunderbird. I have added the relevant senders to the yahoo web address book. I finally chose to use another mail provider. Life is to short for that sort of aggravation.

Well, it's been a week since I went to Yahoo Mail and changed the setting for this particular email, and it has not landed in the TB junk folder since. I consider this problem solved, and will look into Yahoo settings if it happens to another non-junk email. Thanks for the responses.

Can you walk me through the Yahoo Mail settings please, I have the same problem with my Google Calendar email reminders. Have been on to Google, marked as "not junk", capital J, added Yahoo address as well as Gmail address, nagged at Firefox and now at least you seem to have answered this question.