Using Thunderbird as your e-mail client can greatly reduce the amount of junk email that reaches your Inbox. While it cannot prevent all junk mail, the filter gets better over time as it learns what is junk and what is not.

The instructions below will help you setup Thunderbird's built in Junk mail filter

1. In Thunderbird, click on Tools and choose Account Settings.
2. Click on Junk Setting.
3. Place a check mark in each of the boxes. Make sure to trust junk mail headers from Spam Assassin. You can change how long before Thunderbird deletes messages.
   
4. Press OK to finish.

Thunderbird will place this flame icon  next to all messages that it believes to be Spam. 

At first, Thunderbird may have trouble differentiating between your real and junk mail. You will need to correct it when it makes mistakes either way. In time, Thunderbird will do a wonderful job of putting junk mail from your mail folders into your Junk folder.

Use Thunderbird Filters to Organize Your Email

Filters can move messages from specific senders into folders for you automatically, or label messages that contain key words. Organize your e-mail the easy way by following instructions on this page to create your own custom filters.

1. Within Thunderbird choose Tools and select Message Filters.
2. The Message Filters window will open showing you all current filters.
3. Click on the "New" button to create a new filter. This will bring up the Filter Rules so that you can start creating filters. If you want to change an existing filter, choose Edit or you can Delete old unwanted filters. The filters run in priority of the rank they have on your screen. You can move them up or down the list with the Move buttons.
4. Use the drop down menus to program in what Thunderbird matches and what actions it should take for these messages. In our example above, we chose that all messages with the Subject "WU-SPAM" to be moved into the Junk folder. You can add more match conditions or actions by using the + button. The - button will remove the condition or action highlighted. Press OK once your filter is ready. You can close the Message Filters box as well.
5. The filters will work on all incoming mail immediately. To retroactively apply the filter to your inbox, first highlight your inbox folder in Thunderbird, then choose Tools and select Run Filters on Folder. You may also do the same to your other folders.