Earlier quoted context omitted.
If you want to send something securely, I’m not sure email is the best method. Even if you understand all the risks and pitfalls and side-step the minefields it’s still insecure in non-fixable ways. ProtonMail make a service out if it mitigating those issues as best they can (but even they explain their threat-model and what they DON’T mitigate. Others such as Silent Circle ended up giving up. You could recommend to…
Signal is not secure if you're concerned about the App Store maintainer or Signal themselves having access to your data (you can't know what the contents of the app on the app store are and it my exfiltrate keys. This has been done with other similar apps in the past.)
Such as?