Yes email is inherently insecure, don't forget your emails also sit on lots of other systems too usually unencrypted unless everyone else has opsec matching yours. Don't put important secrets in email I guess.

Frustrating that it was designed so badly and I don't think there's a way to fix it without fixing protocols really and doing E2E encryption and signing with keys only on devices, even then you depend on all the code handling that to be secure, and nobody on the other side to store plain text of your encrypted emails (which they must be able to see/get after all).