What is the Tarpit filter (filter rule #17)?
If you let us know what email addresses at your domain are valid, we can provide a certain level of protection against directory harvesting attacks. For example, if an email message is sent to 10 different email addresses at your domain, and at least three of them are invalid, we reject the remaining 7, even if they were to valid email addresses. These 7 email messages were “tarpitted”. This helps prevent the spammer from determining which of the 10 email addresses may have been valid, as we report to the spammer that all of them are invalid.
