That's already cryptographically built into email with DKIM. An email message is signed with a private key, and can be verified as authentic by the receiver using the public key in the sender's DNS records.
Click on "Show Original" in a Gmail mail to see the meta data.
Many email providers historically used short keys (RSA <512, and later 1024) for DKIM, meaning that lots of emails circa 2012 and older appear to be nonrepudiable but in fact are.
Long-term authenticity of emails is very difficult and is arguably an anti-feature; Matt Green correctly observes[1] that Google should be rotating and publishing its DKIM keys regularly to prevent people from falsely concluding that a DKIM signature is "proof" of a message's authenticity.
[1]: https://blog.cryptographyengineering.com/2020/11/16/ok-googl...