Unencrypted inboxes can be accessed by anyone with the Username.

REUSABLE.EMAIL/any-name

Encrypted inboxes and their contents can only be accessed with a valid RSA Private Key, which is generated locally and never recieved by us.

The only setup required is setting an mx dns record.

imap.reusable.email

Custom domains don't get seperate inboxes. Use a 'private' subdomain for encrypted inboxes (ex: private.example.com)