Global rules to drop all other recipients
or set SMTP relayer to your Exchange server (and catch all messages there, bouncing as needed)
as for receiving, just don't allow port 25 connections, block at firewall or edge device or via hmailserver.ini
hMailserver question
Re: hMailserver question
Just 'cause I link to a page and say little else doesn't mean I am not being nice.
https://www.hmailserver.com/documentation
https://www.hmailserver.com/documentation