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Email filters can be set up in webmail and Thunderbird to automatically file messages in specified folders. For instance, you may want to put all your messages from a news group into one folder, all messages from family members into another, and all marked as spam into your junk folder.
Filters make decisions based on conditions that you define. Before you set up a filter, make sure you have a clear idea of what condition you are defining. Are you making a determination based on the sender, the subject, or some other criteria? Where are you going to put the mail that gets filtered?
Once you know what your filter is going to do, the creation process is pretty straightforward.