Movable Type version 3.2, released in April of 2007, included a plugin known as SpamLookup. This plugin was able to submit comments to a lookup service (hence the name of the plugin) to one of a number of blacklist services, which in turn would determine if the IP address used for the comment submission is suspicious, allowing you to better judge if you had encountered a spam comment.
However, way back in May of 2007, Six Apart released a guide on updating your spam filters for the plugin. Without the change, your comment-posting process will be horribly slow, because the process has to timeout before it will continue. Also, the default for the other service probably isn't the best (though that isn't mentioned). Want to know more? Read on!