Postm@ster Pro is a server based anti-spam tool which provides the most effective defense against unwanted email by using Sender Authentication Technology (S.A.T.). Unlike more traditional approaches to spam prevention, there is no risk of erroneously blocking legitimate mail because the software does not depend on message content, i.e. specific words or phrases which may not always be unique to spam. Instead, mail is either blocked or allowed to pass through based on information contained in the email that is unique to each individual sender, i.e. email address, hostname, IP address, originating Class C network. Postm@ster Pro will protect individual webmasters, entire servers, or all the servers on a class C network depending on the licence purchased. It provides a user friendly web interface for each individual email account user. Through this interface, each account holder may enable/disable the software at their option or configure it to their precise filtering needs. The main feature is to place all incoming mail from unknown senders into a queue pending authentication of the sender. Upon authentication, the email is delivered. This functionality may be disabled at the individual user's option in order to allow all mail through unless that user specifically blocks it based on sender's email address, hostname, IP address, or the originating network.