An Account can have simplified Rules to handle junk E-mail messages.
These Rules rely on other Rules, usually employing External Filter programs, to add a special message headers field with the "junk probability" level.
Each Junk Control Rule has a condition checking contents on the X-Junk-Score message header field. The other condition checks if the message From address is not included into the AddressBook dataset ("whitelisting").
If the conditions are met, a Junk Control Rule can either discard the E-mail message, store it in the Junk Mailbox (the Junk Mailbox name as specified using the Account Preferences), or mark the stored E-mail Message with the Junk flag.