Hello everyone First of all I want to apologize if there is already exist same question with answer. I tried to search. no luck I have fully working Debian server with ISPconfig 3.1 and everything seems to doing good But my boss wants me to selectively forbid users to send mail outside of the local mail. (Let's say we have a domain "ourcompany.online" users can send emails to each other but not all users can send emails outside. I think in future my boss will want selectively allow users to send email to certain domain/domains) Is it possible to do? Would be amazing if someone give me a hint or a link how to do this.