I love what your doing with this project but nothing can be more secure than keeping your mailbox @ home. You know instantly if anyone has tampered with it or if someone is dumping your RAM in the case of VPS to get your private key data.
I would love to see a simple script that could be run on a VPS host that sets up iptables to forward all incoming connections on port 25 to the users IP or noip dynamic dns to a port not blocked for example 2525. This would allow external mail servers to transparently connect to your home mail server on a non blocked port.
Additionally when you send email outbound to do the reverse to connect to your VPS via port 2525 or 2526 at which it will then forward your connect via iptables or another proxy based mechanism to the intended mailserver.
I know there are services like sendgrid but they now all have a copy of every email and this defeats the purpose. The idea of the vps is just a dumb iptables type script that lets you bypass home port25 restrictions and lets you utilize TLS 1.2 mail server to mail server if the supported. Please consider this, it could be done via bash or python.
Thanks.