Have local-only mailboxes that can only send mail to or receive mail from their own domain
Gustavo Baetti shared this idea
Gustavo Baetti commented
I have a client who asks me to block some accounts of his domain so that they cannot send emails outside the domain, if they can send to the same domain.
And also that some accounts cannot receive emails outside the domain
This functionality is provided by the ALT-N MDAEMON