First, POPFile is not tied to a certain brand or type of email client or server.
POPFile is written in Perl. As Perl is available for a wide array of platforms, POPFile is compatible with a very wide array of computers.
It is a highly configurable email classifier, not simply a spam filter. Email can be classified to any number of categories you define.
POPFile uses
pseudowords to handle various message elements and spammer tricks to help improve accuracy.
POPFile doesn't use heavyhanded approaches like blacklists or excessively openended approaches like whitelists, both of which can be problematic when they make a mistake.
As a general mail filter, it stands out from the crowd as it has the capacity to learn, and learn quickly.
As a spam filter, it stands out due to its high accuracy and elegance through simplicity in design.