Using the Spam Filter in Mozilla Thunderbird

Thunderbird excels at spam detection

Spam filter

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Open-source email app 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 don't like spam in your Mozilla Thunderbird inbox, you should turn on the junk mail filter.

How the Mozilla Thunderbird Spam Filter Works

The Bayesian analysis Mozilla Thunderbird uses 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.

How to Turn on the Spam Filter in Mozilla Thunderbird

To have Mozilla Thunderbird filter junk mail for you:

  1. Select Options > Account Settings from the Thunderbird hamburger menu.

    Screenshot depicting the Thunderbird account settings menu option
  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.

    Screenshot depicting the Thunderbird junk settings
  3. Click OK.

How to 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:

  1. Open the spam filter settings for the desired email account in Mozilla Thunderbird at Options > Account Settings > Junk Settings.

  2. Make sure Trust junk mail headers set by: is checked under Selection.

    Screenshot depicting junk email settings in Mozilla Thunderbird
  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 unwanted emails, blocking senders does little to fight spam. Junk emails don't come from identifiable stable email addresses. If you block the email address from which one spam email seems to come, there's no noticeable effect, because no other spam email will ever come from the same address.