Caddy I don’t know at all. However, from my experience with WireGuard and iptables (Homemade WireGuard VPN on a VPS server) for port 25 you have to be careful to route it both directions (from internet via VPS to YH and vice versa). I think that currently you don’t do the routing from YH to internet via the VPS.
It seems to be a problem with the routing of port 80. Check in Caddy that you are not forcing traffic to https.