On Debian systems, Exim4 in its default configuration does not allow piping of an incoming mail into a program defined in /etc/aliases. Instead of launching the program it will report this type of error in
system_aliases defer (-30): pipe_transport unset in system_aliases router
Yet that’s the method suggested in manitou-mdx documentation’s “Delivering incoming mail into files” section, and it has the advantage of being quite standard across most MTAs. This is also a problem for popular mail software such as MailMan, as mentioned in this Ubuntu issue.
The solution is similar to the one mentioned in the above issue, except that it’s better to create a custom configuration file rather than editing
exim4.conf.template, according to update-exim4.conf manpage.
In the simplest case where a split configuration for Exim is not used (dc_use_split_config=’false’), the fix is as simple as creating
SYSTEM_ALIASES_PIPE_TRANSPORT = address_pipe
SYSTEM_ALIASES_USER and SYSTEM_ALIASES_GROUP may be specified too if the defaults are not suitable, but only SYSTEM_ALIASES_PIPE_TRANSPORT is strictly necessary.
If a split configuration is used, the line should go into a file under