exim4 and catchall email address

If you search for a way how to configure a catchall email address with exim4, it is highly probable that you will see a router like:

  debug_print = "R: system_aliases for $local_part@$domain"
  driver = redirect
  domains = +local_domains
  data = ${lookup{$local_part}lsearch*{/etc/aliases}}

In this case the catchall mechanism is included in the system_alias router that normally just uses:

 data = ${lookup{$local_part}lsearch{/etc/aliases}}

Thus each email sent to an address entered before the “:” in /etc/alias will be redirected to the mailbox entered after the “:”.
By changing lsearch to lsearch* you can have an entry in /etc/aliases that looks like

*: catchall

This should be at the end of the alias file and for every address that has no other entry, the email is redirected to the catchall-mailbox.

Unfortunately this has the drawback that you need to add an entry for every user that should get emails that looks like:

user: user

If you ommit it, all emails to user will be put into the catchall-mailbox. That’s because the sequence of exim4 routers matters and in the Debian default configuration the router that checks for local users is put behind the system_aliases-router. You might think about changing the sequence of routers, but this is generally a bad idea. If you reverse the order of system_aliases and local_user, you can no longer redirect emails to system accounts like uucp or news to something more appropriate.

So, why not leave the system_aliases-router alone and simply add another router at the end of the router section:

  debug_print = "R: catchall for $local_part@$domain"
  driver = redirect
  domains = +local_domains
  data = catchall

It is very similar to the system_aliases-router but does not search anything for a matching entry but simply redirects everything to catchall. If it is really at the end, the email would have been rejected without this router and so no harm related to the behaviour of other routers is done. Due to driver = redirect it even takes care of .procmailrc and/or .forward …