> Organizations that need to let certain members hear from the general public, or communicate with untrusted (perhaps anonymous) customers, may establish a separate service instance for that purpose.
Rather than throwing away all existing email clients and servers, couldn't this be achieved by configuring an inbound mail rule to drop all email from addresses that aren't in your address book (and from servers that don't support DKIM)?
It doesn't seem very meaningful to say "Users should only receive messages from approved senders, unless they need to receive messages from unapproved senders". Email is useful precisely because it doesn't have the extra step of requiring users to manually add an address to their address book before receiving a message from a new contact, such as a receipt from an online purchase.