The rest of this chapter discusses the various types of UBE checks Postfix provides. It considers four different categories of spam detection which are listed below.
Four parameter rules that work with pieces of the client identity. Each rule is assigned a list of one or more restrictions that can explicitly reject or accept a message or take no position one way or the other (commonly indicated as DUNNO). For example, you can configure a rule that includes a restriction to reject a particular client IP address.
Parameters that check for strict adherence to the standards. Since spammers often don't follow the published standards, you can reject messages that come from misconfigured or poorly implemented systems. Some of the client restrictions also fall under this category.
You can check the headers and the body of each message for tell-tale regular expressions that indicate probable spam.
You can define complex client-detection rules with restriction classes. These allow you to combine restrictions into groups to form new restrictions.
When configuring Postfix to detect spam, you also specify what to do with messages identified as spam. In general, Postfix can reject them outright, separate them into a different queue, or pass them along to an external filter.