"Suppose every mail you receive gets sent through a little program before delivery. This little program does several things. It strips off any attachments, and stores them in an SQL database, adding a note to the end of the email pointing out the ID number of the attachment in the database.
It also stores information about who the mail was sent from, the subject line, the date, some keywords as determined by Text::Keywords, and so on. The add-on "recogniser" modules get hold of the email and try to pull out various things - email addresses, patches, phone numbers, mailing addresses, and so on."