These databases contain lists of known spammers, open relays, hijacked systems, dynamic IPs, and so on. There are various DNSBL projects out there and although they differ in functionality, they use the Domain Naming System (DNS) to create databases that people or products can query. Such databases are also periodically updated. By cross referencing this database, anti-spam software can fairly accurately classify spam emails which contain any of these elements. These databases often include information such as known spammer sources, finger printing data, known spam links, spam images and attachments. Some vendors have compiled databases of detailed information that can be used to detect spam. If the email was sent from a source not present in that list it is rejected as spam. The SPF record includes a list of authorized hosts that can send emails for that domain. SPF detects this by checking the email’s SPF record for the domain of origin. SPF detects spam by assuming that many spam emails lie about their true origin. Spam emails are normally sent from mail servers that do not follow this protocol and are therefore filtered because they have not been resent. This technique relies on the email being sent from a proper mail server which follows set standards, and that server will then resend the email a short while later. Greylisting is a process whereby an email from an unknown source is rejected with a temporary error. Understanding how these methods work can help you to decide which anti-spam solution will be the most effective. If humans sometimes find it difficult to tell the difference between a spam and a legitimate email, how can a computer program accomplish this? There is a variety of techniques that anti-spam software uses to keep your Exchange server clean. However, this is not as easy as it sounds. You want an anti-spam solution that blocks all spam emails, while allowing legitimate email through. Given the wealth of options available, choosing which solution is best for your needs can be a challenge.
Protecting your Exchange server infrastructure from email spam can be easier said than done. An In-depth Analysis of Anti-Spam Techniques to Protect Your Exchange Server
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