An organization can add spam filtering by building an email gateway. This is done by using a new machine installed with Linux, SpamAssassin, Postfix, and Amavisd-new.
The email filtering can take place without any changes to the existing mail server. Spam filtering can be enabled and disabled by directing existing traffic to the old email server or the new email gateway as required. Email is tagged for spam, and a threshold can be set where spam email is simply discarded.