Setting up wildcards with Exim4 and virtual domain

This is a quick howto for setting up Exim4 with:
1) several incoming (virtual) domains
2) some local users
3) wildcard catchall accounts to collect the emails not matching the local users

• Setup the virtual domains in the dc_other_hostnames variable in /etc/exim4/update-exim4.conf.conf. These are the domains considered local by this installation of Exim4. Any other domains will be routed as remote mail either through the IP defined by dc_smarthost, or direct delivery through DNS lookup of the MX records, or a hubbed_hosts definition file if it exists which overrides the DNS lookups.
  
• Create the local users with adduser
• If required, create the local user sending addresses (for the From: header) in /etc/email-addresses (otherwise the local user will be stamped with the mail hostname defined in /etc/mailname)
• #/etc/email-addresses
  
• user1: user1@domain1.com
• user2: user2@domain2.net
• Define the catchall accounts in a new a wildcard aliases file /etc/aliases_wildcards
• # /etc/aliases_wildcards
• #*@domain1.com: domain1-catchall-user
• #*@domain2.net: domain1-catchall-user
• Create a new router directive to process the wildcard aliases - the first 3 digits of the filename define which order routers are processed by Exim4. This needs to be after the local_user router which is by default 900 so any value higher than 900 will work. I've chosen 999. /etc/exim4/conf.d/router/999_exim4-config_system_aliases_wildcard
• # This router handles wildcard aliasing using a /etc/aliases_wildcards file
• # /etc/aliases_wildcards must exist as it is a non-standard file
• system_aliases_wildcards:
• debug_print = "R: system_aliases_wildcards for $local_part@$domain"
• driver = redirect
• domains = +local_domains
• allow_fail
• allow_defer
• data = ${lookup{*@$domain}lsearch*{/etc/aliases_wildcards}}
• file_transport = address_file
• Check the routing works as expected
• exim4 -bt user1@domain1.com
• exim4 -bt notauser@domain1.com
• The first command should show email routing to a local user using the local_user router.
  
• R: system_aliases for user1@domain1.com
• R: userforward for user1@domain1.com
• R: procmail for user1@domain1.com
• R: maildrop for user1@domain1.com
• R: lowuid_aliases for user1@domain1.com (UID 1001)
• R: local_user for user1@domain1.com
• user1@domain1.com
• router = local_user, transport = maildir_home
• The second command should show email routing to the catchall account again using the local_user router:
• R: system_aliases for notauser@domain1.com
• R: system_aliases_wildcards for notauser@domain1.com
• R: system_aliases for domain1-catchall-user@localsystem.com
• R: userforward for domain1-catchall-user@localsystem.com
• R: procmail for domain1-catchall-user@localsystem.com
• R: maildrop for domain1-catchall-user@localsystem.com
• R: lowuid_aliases for domain1-catchall-user@localsystem.com (UID 1000)
• R: local_user for domain1-catchall-user@localsystem.com
• domain1-catchall-user@localsystem.com
• <-- notauser@domain1.com
  
• router = local_user, transport = maildir_home
  

A good source of information on which config file does what in Exim4 can be found here