Using the Spam Filter in Mozilla Thunderbird

Thunderbirds excels at spam detection

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Open-source Mozilla Thunderbird includes highly efficient spam filters using Bayesian statistical analysis. After a bit of training, its spam detection rate is stellar, and false positives are practically nonexistent. If you do not like spam in your Mozilla Thunderbird inbox, you should turn on the junk mail filter.

Turn on the Spam Filter in Mozilla Thunderbird

To have Mozilla Thunderbird filter junk mail for you:

  1. Select Preferences > Account Settings from the Thunderbird hamburger menu.
  2. For each account go to the Junk Settings category under the desired account and make sure Enable adaptive junk mail controls for this account is checked.
  3. Click OK.

Prevent Mozilla Thunderbird From Overriding External Spam Filters

To have Mozilla Thunderbird accept and use spam filtering scores created by a spam filter that analyzes messages before Thunderbird receives them—at the server, for example, or on your computer:

  1. Open the spam filter settings for the desired email account in Mozilla Thunderbird at Preferences > Account Settings > Junk Settings.
  2. Make sure Trust junk mail headers set by: is checked under Selection.
  3. Select the spam filter used from the list that follows.
  4. Click OK.

Blocking Senders Doesn't Help

In addition to employing a spam filter, Mozilla Thunderbird lets you block individual email addresses and domains.

While this is a proper tool to avoid senders or automated software installations that keep sending emails in which you have no interest at all, blocking senders does little to fight spam. Junk emails do not come from identifiably stable email addresses. If you block the email address from which one spam email seems to come, there is no noticeable effect because no other spam email will ever come from the same address.

How the Mozilla Thunderbird Spam Filter Works

The Bayesian analysis Mozilla Thunderbird does for spam filtering assigns a spam score to each word and other parts of an email; over time, it learns which words typically appear in junk email and which appear mostly in good messages.