Usually, POPFile relies on the words it finds in email messages to do its classfications. But there is more to an email than just the words. For example, if a word is found in the subject, this should be noted.

Pseudowords were created to store just this kind of meta-information. You can identify pseudowords by their special syntax: pseudoword:value. A list of all the pseudowords used by POPFile can be found on the What are the Pseudowords built into POPFile and how do they work? page.