Thank you! It never occurred to me to consider haproxy to do something like this and I think it is exactly what I am looking for. It's definitely worth further exploration. My subscription for email service is about to run out and I'd rather not pay for something I don't have to.

You can do TCP proxying with nginx but many of the same features available in haproxy are behind the paywall. In nginx, layer 4 connections are dealt with through streams. You can do both TCP and UDP. I stick with haproxy for TCP streams with very few exceptions. HAproxy is most definitely more robust for situations where you have a pool of upstream servers. For single upstream instances, it’s not terrible. Most of the features I would use for better control of how the failover and balancing would work isn’t available in the open source nginx.

I will use my VPS as an SMTP smart host because I have a residential dynamic DNS connection and many mail providers block those IPs by default. I have to see if my Oracle always free tier will let me use port 25. I have a feeling I may bit shit out of luck on that front.

I actually considered doing port forwarding and NAT for this but I would run into a problem with NAT reflection. I'd have to implement split-brain DNS to avoid this. It's more efficient to simple proxy the connection. If I am unsuccessful, however, I will resort to port forwarding.