What I feel is that you need do is to compare the cost of power supply vs hardware then choose whichever is cheaper.
A good read about self hosted email servers: https://news.ycombinator.com/item?id=32715437
If you are going to use it within your family members it will be fine, but not with other users who uses corporate provided email services.
It’s a good tip, but nowadays a lot of apps accessing the contacts. Storing secured information on contacts is vulnerable. Please be mind it.
Wireguard + pihole?