I thought that was the point, since you didn't trust other people not to be mining your data? If it's another person's machine then they could just read your emails...

No, you self-host because you retain the ownership and control of your data. You choose service providers whose main business is not to snoop your data. For example, if you choose a server from amazon ec2 or linode or your local vps provider, they are not going to snoop your data because they are in the business of selling servers and not mining all your servers.

Okay, different question: any reason to believe the government isn't making them share dumps of their servers behind your back? I thought paranoia about the government reading your emails was the other huge reason to want this sort of thing in the first place.

If you're that worried then you can encrypt your server or data folder if needed.

What's the point though? Wouldn't the key need to be on the server if I encrypt it? Otherwise how can my mail server even work?