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<TITLE>Exim Filter Specification - Introduction</TITLE>
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<H2><A NAME="SEC2" HREF="filter_toc.html#TOC2">Introduction</A></H2>
<P>
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 <TT>`.forward'</TT> 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 `<TT>`.forward'</TT>
with conditions'. Operating the set of rules is called <EM>filtering</EM>, and the
file that contains them is called a <EM>filter file</EM>.
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<P>
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.
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<P>
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.
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<P>
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.
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