@kkarhan sounds good, but would fail... Same reason I don't use WhatsApp, signal, etc... People won't use it, and the second you don't get an important email from a customer or time sensitive email, they'll go elsewhere for email or have an email for 'unencrypted' comms, and you already lost.
I used hushmail for years because it allowed for encryption of emails... Except I paid for a service no one uses..
Encrypt the attachment if you need to, send the password in another way, like SMS