There is an error when you have a domain a.com on server 1 and try to add a routing for the same domain on server 2. This error message shall get displayed only when someone tries to add a.com as an email domain and routing on the same server, and not on different servers. https://forum.howtoforge.com/threads/possible-bug-ispc-3-3-1p1-debian-13-email-routing-error-the-domain-is-already-set-as-a-e-mail-domain.95137/#post-470809