[exim] Receive Mail From a Secondary-MX Proxy

I am thinking about how I’ll manage to send and receive mail from the Internet by way of a proxy, with SOCKS for outbound and proxy-protocol for inbound mail, where the proxy is also potentially a backup MX. The idea is that I will run the mailer on a network with a dynamic connection, and use a VPS with a fixed IP for connectivity that’s trustworthy for other MX hosts, which only accepts mail when the ISP connection goes down for a noticeable period. I could even extend the client connection through a VPN, so the ISP is oblivious to how it’s used (the country in question has a very flexible approach to civil liberties).

The doc says Exim recognises a proxy host by IP; does this mean I can’t receive ordinary mail from it as a secondary MX? If not, how do you think I ought to go about this?

What about if I extent this setup so that my mailer machine only makes outbound connections to the proxy host—can I still receive inbound mail, through a forwarded port perhaps? SSH seems like the obvious answer, but then I’d lose sender information, yes? I could use an inner VPN, perhaps. But something that only carries application-layer traffic would be nicer. Exim supports SOCKS, but not the bind method—perhaps that would be useful.

Any thoughts appreciated.

Cheers,
Sabahattin