To deal with large amounts of unsolicited email ("spam" or "junk mail"), Thunderbird uses an adaptive filter. This "adaptive" filter requires training data, which comes from you marking received messages as legitimate (not spam) and which are spam. The filter then identifies new messages it thinks are spam and moves them to the Spam folder.

Overview

Thunderbird's adaptive filter uses a Bayes algorithm with training data, based on messages you mark as spam and not spam. But using an algorithm (a formula) means a message previously marked as spam is not guaranteed to be automatically marked as spam in the future. Likewise, marking a message as not spam does not guarantee a similar message will not be marked as spam. Therefore:

• It is important that you do not delete spam messages, but instead mark them as spam so that training occurs.
• To guarantee that a message WILL be marked as spam, see Create filters manually.
• To guarantee that important messages WILL NOT be marked as spam, use Whitelisting.
• You must frequently train Thunderbird, for Thunderbird to do a good job of identifying spam.

Spam filter settings

Global spam settings

Spam filtering is enabled by default. And you can set global spam settings to tune what should happen to messages marked as spam. These settings will be used for all email accounts (although some settings can be overridden in the Per account settings, as shown below).

1. Click ≡ > Settings > Privacy & Security
2. Scroll down to Spam. Adjust the settings as you see fit.

   

Per account settings

The spam settings in the account settings for each of your email accounts will override similar settings in the Global settings described above.

• Click ≡ > Account Settings > Your email address > Spam Settings

  

How to turn off Thunderbird's adaptive filtering

• Untick Enable adaptive spam mail controls for this account

Whitelisting

Under Do not automatically mark mail as spam if the sender is in you can select address books which will be used as a whitelist. Senders whose email address is in a whitelisted address book are exempt and will not be automatically marked as spam by Thunderbird. But you can still manually marking a message containing a whitelisted address as spam.

It is recommended that you enable whitelisting to help ensure that messages from people you most care about will not be marked as spam.

Training the spam filter

In order for Thunderbird's spam filter to be effective, you must train it to recognize messages that you consider to be spam and messages that you consider to be not spam. If you do not do both, then the filter will not be very effective.

Note also, it is important to mark messages as spam before deleting them. Just deleting a message doesn't train the filter.

Tell Thunderbird what IS spam

There are several ways of marking messages as spam:

• You can click on the little icon in the Spam column of the Message List Pane:

  

• You can also click the Spam button on the Message Header Pane:

  

• Or you can press J on your keyboard to mark one or more selected messages as spam.

Tell Thunderbird what IS NOT spam

Sometimes Thunderbird's spam filter might get it wrong and mark good messages as spam. Therefore, it is just as important to tell the filter which messages are not spam, especially on a new installation of Thunderbird.

There are several ways of marking messages as Not Spam.

• Click on the Not Spam button in the yellow spam notification below the message header in the Message List Pane:

  

• Click on the red spam icon in the Spam column of the Message List Pane to toggle the spam status of a message:

  

• Or press Shift+J on your keyboard to mark one or more messages as Not Spam.

Repeated training

You should regularly train the filter by marking a number of good messages as not spam - messages in your Inbox and messages that have been filtered into other folders. In this case, you can only use the keyboard shortcut Shift+J, because the Not Spam button appears only for messages that have already been marked as spam. Marking several messages per week will be sufficient, and you can select many messages and mark all of them at the same time.

Unfortunately the user interface does not help you see whether a message has already been marked as "not spam".

Other ways of blocking unwanted messages

Thunderbird's adaptive spam filter is not an absolute barrier against messages from a specific address or specific types of messages. You can have stronger mechanisms to stop unwanted messages by using the following methods.

Create filters manually

You can manually:

• Block some senders.
• Create filters to block messages based on subject or other criteria.

Use a Thunderbird supported external filter service

If your mail provider offer SpamAssassin, Bogofilter, DSPAM, POPFile or SpamPal, then you can use their external filter service to help classify email and block spam.

1. Click ≡ > Account Settings > Your Account > Spam Settings
2. Enable the Trust spam headers set by option.
3. Choose an external filter service in the drop-down menu.

   

Use your mail provider's spam or filter features

Your mail provider may offer features for avoiding spam or blocking messages.

Also, mail providers often use services which collect data from thousands or millions of users to provide superior spam filtering. Depending on your provider, you can assist in the collection process by moving spam messages to the Spam folder or by marking messages as spam using the mail provider's webmail.