Exim is highly configurable, and therefore has features that are lacking in other MTAs. It has always had substantial facilities for mail policy controls, providing facilities for the administrator to control who may send or relay mail through the system.
In version 4.x this has matured to an Access Control List based system allowing very detailed and flexible controls. The integration of a framework for content scanning, which allowed for easier integration of anti-virus and anti-spam measures, happened in the 4.x releases. This made Exim very suitable for enforcing diverse mail policies.
