Unfortunately involving a server based hosting solution seems rather death knellish. That automatically puts a end to feasibility unless a large amount of players continue to play AoW3. With current player numbers it leaves me to believe in 1.5-2 years they will have to shutter PBEM entirely or have some sort of subscription charge.
I wish they would put a p2p pbem so if you want to play with a well known friend and you are both on good email services you could still be using PBEM in 2021 when there are only a few hundred people who still play AoW3. But with the mandatory login to play the game, I suppose we may not be able to do any MP period at that point.