When an SMTP connection comes in, you don’t know who it’s for (RCPT TO) until after the STARTTLS handshake, so you wouldn’t know whose phone to forward that traffic to. The only reliable way to do that would be a dedicated IP address for every user. Since SMTP is pretty much exclusively IPv4, that would be expensive. Not impossible, but expensive. If you’re doing it for only a handful of users, it could definitely work.
When an SMTP connection comes in, you don’t know who it’s for (RCPT TO) until after the STARTTLS handshake, so you wouldn’t know whose phone to forward that traffic to. The only reliable way to do that would be a dedicated IP address for every user. Since SMTP is pretty much exclusively IPv4, that would be expensive. Not impossible, but expensive. If you’re doing it for only a handful of users, it could definitely work.
Also, thank you for the compliments. :)