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There is none. Email is insecure by design and it will always remain that way. Even if you develop something secure (whatever that may be), you still have to communicate with other email providers that are not gonna implement your security features.

GPG is a slight improvement that you can use on top of any provider and it will encrypt the content of emails (and content only, metadata will still be there and even subject lines in certain implementations), but both you and the recipient will have to use it and good luck convincing non-IT people to deal with its absolutely atrocious user experience.

In other words, treat it like a physical mailbox. Good enough for various notifications, useless if you're trying to protect anything valuable in it.