Most Unix mail transfer agents (programs that deliver mail) permit individual users to specify automatic forwarding of their mail, usually by placing a list of forwarding addresses in a file called .forward in their home directories. Exim extends this facility by allowing the forwarding instructions to be a set of rules rather than just a list of addresses, in effect providing ``.forward with conditions''. Operating the set of rules is called filtering, and the file that contains them is called a filter file.
The ability to use filtering has to be enabled by the system administrator, and some of the individual facilities can be separately enabled or disabled. A local document should be provided to describe exactly what has been enabled. In the absence of this, consult your system administrator.
It is important to realize that no deliveries are actually made while a filter file is being processed. The result of filtering is a list of destinations to which a message should be delivered - the deliveries themselves take place later, along with all other deliveries for the message. This means that it is not possible to test for successful deliveries while filtering. It also means that duplicate addresses generated by filtering are dropped, as with any other duplicate addresses.
This document describes how to use a filter file and the format of its contents. It is intended for use by end-users. How the system administrator can set up and control the use of filtering is described in the full Exim specification.