To deal with the large amount of unsolicited email ("spam" or "junk mail") that most people have to cope with, Thunderbird uses an adaptive filter that learns from your actions which messages are legitimate and which are junk.
The Junk filtering does not have regard to the senders email address. Junk mail frequently uses legitimate and known sending addresses which are changed regularly so they are not a good indication on unsolicited commercial email.
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Junk filter options
The filter is enabled by default. You can set system-wide preferences that determine what happens to messages marked as junk. These settings will be used by all your email accounts (although some settings can be overridden in the account settings, as shown below). To access the preferences, click, select the panel and then select the tab.
Per account settings
The configuration in the account preferences for each of your email accounts will override similar settings in the preferences described above. Clickand select in the left pane for the account to see these settings. This section includes the ability to enable addressbooks which will be used as a whitelist. If the sender of a message is a contact in an addressbook that has been enabled for whitelist, then that message will not be marked as junk.
Training the junk filter
Tell Thunderbird what is junk
In order for this filter to be effective, you must train it to recognize the messages that you consider to be junk and the messages that you consider to be not junk. So you will want to mark messages as junk, not delete them.
You can mark messages as junk by clicking in the "junk" column in the message list:
You can also mark messages as junk in the message pane by clicking the junk icon on the message header toolbar:
You can also use the small case j key to mark messages as junk.
You must mark many messages so the adaptive filter has enough training data, including messages that are NOT junk. (more about that below)
Tell Thunderbird what is NOT junk
It as just as important to tell the filter which messages are not junk.
First, frequently, daily or weekly, check your Junk folder for good messages that have been incorrectly classified as junk by clicking on the 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.button, or using the upper case
Secondly, you must constantly train the filter by marking a quantity of GOOD messages as not junk - messages in your Inbox AND messages that have been filtered into other folders. 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".
If you still have trouble blocking messages
The adaptive junk filter is not an absolute block against mail from a specific address or specific type of message. If you need a stronger mechanism to stop unwanted messages, then consider Blocking a sender. You can also create a filter to block subject content.