https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

We cannot verify a single email address, but here are some suggestions: - check your online account, as some email providers do their own spam checking - mark that message as 'not spam' when it appears and also periodically mark other messages, both good and bad. That is how thunderbir learns - for a particular email address that consistently fails, I suggest creating a filter to bypass spam checking. This URL may also help. I agree, spam is sometimes difficult to stop. https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages