I don't know how this is actually implemented, but it would be possible to do this as described if the static IP provided by Helm just proxied all incoming pack

I don't know how this is actually implemented, but it would be possible to do this as described if the static IP provided by Helm just proxied all incoming packets directly to the personal server, without acting as a relay server at the SMTP protocol level but instead just being a dumb proxy. There's no reason that Helm would need to act as a mail hop at the SMTP layer or do TLS termination anyways. The reason that the using the server's actual IP wouldn't work is that most residential IP ranges are responsible for mostly spam (so most outgoing email would likely be filtered), and some ISPs don't assign static IPv4 addresses to residential users (IPv6 would fix the later issue, but mass IPv6 adoption isn't going to happen for a while).