At the same time I often get mail that, while it is interesting and important, can easily be filed to be read later, without ever actually having to clutter up my incoming mailbox.
This, then, is what filter does! The filter program allows you to define a set of rules by which all incoming mail is screened, and a subsequent set of actions to perform based on whether the conditions are met or not. filter also has the ability to mail a summary of what actions it performed on the incoming mail as often as you'd like.