Email uses tls. If both ends support it, it uses it. Most popular email providers support it. Also, dmarc signs emails the guaranteeing domains, and if you want email to be reliable, you must support dmarc.
And furthermore TLS only protects the email between hops.
But yes, email is not the problem. The problem is of course human error and legacy systems that are not user friendly. If they were properly composable then there would be no need for occasional out-of-band channels for sending/receiving invoices - like email.