To do what you want, both machines must be on the same network - and having what you consider a high-risk machine (which isn't getting updates) and your main machine on the same network is... inadvisable.
A better idea if you want to fully wall off email because you are a high-risk individual is to use a cloud PC/VM, and in addition use the high-disk individual protection services offered by Apple, Google, or whichever email service you're using.
It all depends on if you're actually high-risk, all the way down to just a 'i know better' tinfoil hat or just a plain paranoid who's making their life more complex than it needs to be.
If the latter, then just get yourself a google workspace account and avail yourself of nearly the best spam filtering service out there on the internet.