Como a página de suporte da Mozilla Mensagens do Thunderbird e Junk / Spam estados
Tell Thunderbird what is NOT junk
It as just as important to tell the filter which messages are not junk.
First, during the early learning stages you will want to frequently, perhaps daily, check your Junk folder for messages that have been incorrectly classified as junk by clicking on the Not junk button, or using the upper case J on your keyboard. After the first week you should also check the junk folder for messages that are incorrectly marked as junk, perhaps weekly.
Secondly, very important, you will want to constantly train the filter by marking a quantity of GOOD messages as not junk, for example messages in your Inbox. You must use the keyboard upper case J, because there is no button - the "Not Junk" button appears only for messages that have already been classified as junk. Marking several messages per week will be sufficient. You can select many messages and mark them all at the same time. Note - unfortunately nothing in the user interface indicates whether a message has already been marked as "not junk".
Dê uma olhada no restante da página também, há algumas dicas interessantes.