Yeah but this way you can only communicate with other Tor users. Also the mail server can prolly easily leak your actual IP if you don't harden it well.
The .onion means it's an onion service. This requires a Tor connection both for the host and the users. And frankly, hosting an onion service is not exactly easy to do it right, due to additional security considerations. Many self hosting guides assume a typical residential connection or a rented VPS without an onion service.