you forgot the US Government spying. until our IT giants tell the US government that they are leaving the united states if they don't stop, there is no reason to continue to freely use their service when an alternative is available.
What alternative? You have to assume any US based email provider is the same when it comes to spying. The big guys in other countries may be safer if:
1) their NSA equivalent isn't doing the same thing.
2) Their spy agency isn't cooperating with/owned by the NSA.
3) the NSA ( and their spy agencies.) can't just tap into the lines that all your email go over.
The small players (in any country) might be safer if they can stay under the radar, but they will *probably* give in faster. I would imagine that Google/Microsoft etc. have bigger legal departments than many small to medium businesses have staff.
I suspect your only option is end to end encryption with the server running on your hardware in your home or office. And hope that you stay unnoticed so that no one sends out the cops on a trumped up weapons warrant, who just happen to decide that your email server looks interesting.
Paranoid, maybe, but a $3 wrench trumps most kinds of security. (allusions intenionaly not linked)