You make very good points. Corporate environments generally have an easier time with s/mime, and IIUC this app is focused on corporate environments. GPG is ni

You make very good points. Corporate environments generally have an easier time with s/mime, and IIUC this app is focused on corporate environments. GPG is nice (I like a lot and have used it for a very long time), but it's certainly niche. That said, I included s/mime and gpg to illustrate that end to end encryption is not only possible, but that mature, functional tools are available in an SMTP+extensions environment. It would be interesting to know how mnm/TMTP does encryption. I'd assume it uses TLS for transport. As for endpoint and server storage encryption (assuming it does that at all), you'd hope they'd use some sort of asymmetric key encryption, which raises the same issues as s/mime and gpg.