I no longer consider any email app to be okay for privacy if I can’t build it from source code. There are just too many opportunities and incentives for someone to exploit it. That could be the developer, or the maintainer of some obscure code library, or a company that buys one of them out, or an attacker who found a vulnerability. We no longer live in a world where it’s reasonable to think we’ll get privacy from communications software that we can’t inspect.

Thankfully, we also no longer live in a world without options. There are more than a few email apps with nothing to hide. :)