So our goal is: All outgoing mails to a specific domain should be delivered normally (e.g. your company domain) while all other outgoing mails should be delivered to one specific catch-all mailbox.

Actually this task can be resolved by adding not more than three lines of configuration code in two files.

1. in /etc/postfix/main.cf add

virtual_alias_maps = pcre:/etc/postfix/virtual.pcre

2. the new /etc/postfix/virtual.pcre should look like

/(.+)@yourowndomain\.tld$/ ${1}@yourowndomain.tld
/(.+)@(.+)/ catchoutgoing@yourowndomain.tld

After having restarted postfix you are already done.

Please note, a ${0} instead of ${1}@yourowndomain.tld did not work for me, so this a tiny workaround which does not look beautiful but really does it’s job.

p.s.: A really nice (but somehow old looking) tool for developing and testing regular expressions is “redet“, a “regular expression development and execution tool” which knows nearly every important regex syntax depending on the tools you have installed. You’ll find redet in your Debian/Ubuntu repository.