How can an ISP reduce the amount of spam their customers get?
1.4.2.1 Stop Accepting All Email This will immediately reduce the spam intake of their customers to zero. Unfortunately, it also destroys email as a usable communication medium. In order to prevent this becoming necessary whilst still taking action to reduce their customers’ spam levels, many ISPs adopt policies that are midway between blocking everything and doing nothing… 1.4.2.2 Filtering One tactic used by some ISPs to cut down on spam is filtering. The ISP scans incoming mail and any messages that match the pattern of a known piece of spam are discarded. The big danger with filtering is that of false positives; users are unlikely to be very pleased if some non-spam mails are mistaken for spam by the filter and never arrive. Some of the filtering techniques discussed in 1.4.1.6 can also be applied across an entire I.S.P., although there may be additional risks due to questions of scale.
