Running a mail server nowadays is no easy task with the flow of junk mail that we receive. There are several ways used to filter spam ranging from content filtering to an outright block on whole countries.
A blacklist or DNSBL is generally a list of IP addresses from which spam originate. The mail server does a DNS query to determine whether the computer connecting to it is listed and if so rejects the mail. Each blacklist has its own listing policy. Some blacklists include all IP addresses from the Asia region as a sizable amount of spam originates from China and Korea.
A blacklist may also list partial hostnames from dial-up blocks.
Greylisting is a technique used to temporarily reject connections from a host for the first connection or on email address and IP address tuples. We greylist Windows computers as they are usually infected with viruses and other spam generating malware.
The contents of the email message is inspected for tell-tale signs of spam and the message is assigned a score. Messages over a pre-set score are flagged as spam and may be rejected by the mail server.
Whitelisting allows you to bypass the usual anti-spam measures and accept email from a remote computer. You can download our SpamAssassin plugin to whitelist mail.
The sender address is commonly forged in spam and virus infected emails. Sending bounces to these addresses causes backscatter.http://www.elandsys.com/resources/antispam/index.html