Slamming Spam: A Guide for System Administrators

Static Filters

One of the first methods available to block spammers was to parse the inbound SMTP connection MAIL FROM field and compare a static list of known spammers against that field. If the username and/or domain attempting a connection was contained on that list, then the message was rejected. If the username/domain in MAIL FROM line was not on that list, then the message was allowed to continue through subsequent checks. Rejecting mail in this manner is often called “blacklisting.” This mechanism can also be used to allow certain addresses through, otherwise known as a “whitelist.” The list of MAIL FROM addresses allowed or denied is usually a static file generated by the mail system administrator and placed into files for comparison ...