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procmail is a program to filter mail, which can be invoked by sendmail or postfix on your behalf, or called by you directly from a .forward file, or even invoked manually, or via your GUI in some cases.
procmail lets you specify pattern matches on the mail, and decide what to do with it. For instance, if a spam detector tags all your mail with X-Spam-Status: then if the value is set to YES you can use procmail to file the mail into a spam folder.
procmail is increadibly powerful. it also includes the 'formail' program which can for(mat) mail in receiving it: adding and removing headers, changing the to: and cc: details.