https://runbox.com/why-runbox/email-privacy/
Edited to apologise: The cheapest option seems to be $20pa, sorry. I find it excellent value.
But you also don't want to pay any money for it?
Despite what you may have "learned" from the examples of Facebook and Twitter, companies giving away things for free isn't a very good business model.
Stopping a government is different. Are you preventing just the slurping of data? (Just because they have your emails in their cache doesn't mean they're grepping for you); or might you be targeted? In which case you're going to have to carefully use operating procedures and software. And make sure the people sending you email do the same. Also, you'll need better locks, and no windows on the ground floor and etc.