Spam sent to users is blocked using a challenge-response spam filter, linked to the user's own address book. When the user receives an email from someone he or she doesn't know, the server sends an email back to the sender asking them to verify that they're not a spam-bot. This keeps everything as simple as possible for the Celery user. The only special thing he or she need do is make sure to add any mailing lists they join to the address book.