Blackhole Spam/Virus Filter provides spam and virus filtering using dot files in all the major MTA/SMTP servers for UNIX and similar systems. It also can be used as a qmail-queue replacement or a Postfix Content Filter to cover the entire SMTP servers userbase. It is different from other, similar programs, in that it scales for thousands of users from being in C and not executing external programs for most of its work. It is uses flat config files or MySQL to store the configs of each user. Almost every type of check in existence for spam is implimented. It can be setup like an access list with each check type (over 15 checks currently) having its own separate rules for action and scores to choose action precedence. Subject tagging, custom headers, and detailed report logging can be fully configured from the config file, there is a global config file for default users setup, and it works with clamscan, which gives a fully Free spam/virus solution which matches any commercial offering.