Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. In style it is similar to Smail 3, but its facilities are more general. There is a great deal of flexibility in the way mail can be routed, and there are extensive facilities for checking incoming mail. Exim can be installed in place of sendmail, although the configuration of exim is quite different to that of sendmail.
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Example configuration for greylisting using Exim
This package contains a simple example of how to do greylisting in Exim's ACL configuration. It contains a cron job to remove old entries from the greylisting database, and an ACL subroutine which needs to be included from the main exim.conf file. To enable greylisting, install this package and then uncomment the lines in Exim's configuration /etc/exim.conf which enable it. You need to uncomment at least two lines -- the '.include' directive which includes the new ACL subroutine, and the line which invokes the new subroutine. By default, this implementation only greylists mails which appears 'suspicious' in some way. During normal processing of the ACLs we collect a list of 'offended' which it's committed, which may include having SpamAssassin points, lacking a Message-ID: header, coming from a blacklisted host, etc. There are examples of these in the default configuration file, mostly commented out. These should be sufficient for you to you trigger greylisting for whatever 'offences' you can dream of, or even to make greylisting unconditional.