Nostr messages are inherently public though, right?
You can add end to end encryption like with DMs, but others will still be able to see who, when and how often you've been messaging.
Discussion
Keep the email under smtp or what ever the email protocol is. But wrap the email client with a nostr based address book.
Like if I zap you a dollar for example. My email address gets white listed on your contacts list for 1 email.
Once it receives that email, your client removes my email address from the whitelist.