There are a number of appliances, services and software systems that e-mail administrators can use to reduce the load of spam on their systems and mailboxes. Some of these depend upon rejecting email from Internet sites known or likely to send spam. Others rely on automatically analyzing the content of email messages and weeding out those which resemble spam. These two approaches are sometimes termedblocking and filtering.
There is an increasing trend of integration of anti-spam techniques into MTAs whereby the mail systems themselves also perform various measures that are generally referred to as filtering, ultimately resulting in spams being rejected before delivery (or blocked).
Many filtering systems take advantage of machine learning techniques, which improve their accuracy over manual methods. However, some people find filtering intrusive to privacy, and many e-mail administrators prefer blocking to deny access to their systems from sites tolerant of spammers.
A number of systems have been proposed to allow acceptance of email from servers which have authenticated in some fashion as senders of only legitimate email. Many of these systems use the DNS, as do DNSBLs; but rather than being used to list nonconformant sites, the DNS is used to list sites authorized to send email, and (sometimes) to determine the reputation of those sites. Other methods of identifying ham and spam are still used. The A&R allows much ham to be more reliably identified, which allows spam detectors to be made more sensitive without causing more false positive results. The increased sensitivity allows more spam to be identified as such. Also, A&R methods tend to be less resource-intensive than other filtering methods, which can be skipped for messages identified by A&R as ham.
