You could even allow only emails to the same domain (handled on the same server itself). Then emails never leave the server, and people can access them with the

You could even allow only emails to the same domain (handled on the same server itself). Then emails never leave the server, and people can access them with their favourite tool, such as web or IMAP/SMPT, which encrypts everything in transit these days, if set up properly, unless I'm mistaken.