To configure the spam filter for a mailbox:
Note: To further improve spam filter accuracy, you may want to train your spam filter on e-mail messages you receive (see the instructions on improving accuracy of spam detection below).
A network is specified by its starting IP address in the first four fields of the Network/Mask field group. The fifth field is for specifying network mask. It should be a number ranging from 1 to 32, which shows how many higher bits set to '1' the mask contains. For example, for the mask 255.255.255.0 you should specify 24 as the fifth parameter.
The relay hosts on trusted networks are considered not to be potentially operated by spammers, open relays, or open proxies. A trusted host could conceivably relay spam, but will not originate it, and will not forge header data. DNS blacklist checks will never query for hosts on these networks.