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