Crazy idea --- is there a reason you couldn't run an email server as a private messaging system --- no forwarding to other servers? Clients/users will obviousl

Crazy idea --- is there a reason you couldn't run an email server as a private messaging system --- no forwarding to other servers? Clients/users will obviously have to be configured with a private email address/account that only exists only on your server. Does this exist? Has anyone tried/done this?