Hello,
Fastpass is cool technology that improve performance for mails from verified senders. After you enable a database is using and developing automaticly with your clean emails. These performance is coming from that all these good mails are bypass from spam engines. By default it is disabled and it may sometimes make security problems like don't scan a real spam mail even if not coming from good sender.
I don't know if somebody really need technology because Brightmail already deliver messages too fast after filter them all :)
By the way if you really want to use, be sure to check it's advanced settings.
Regards,
Oykun