How can I allow the use of relative paths in users' filter files when the directories concerned are not available from the password data?

You need to be running Exim 4.11 or later. You can then specify a value for $home by setting the router_home_directory option on the redirect router. For earlier releases, there is no way to specify the value of $home for a redirect router; it either comes from the password data as a result of check_local_user, or is unset.

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