ah yes, the dilemma with a trusted or semi-trusted system. what happens when that trust is lost? this is why trustless systems should be preferred. you'll notice while the benevolence of your trusted provider pleases you now, this may or may not remain to be true in the future!
also, remeber that while one (mail) provider may respect your privacy, another may not. so any emails you send to not-so-much privacy respecting providers are compromised.
they say: "if you have nothing to hide, then what's the problem?!"
which to that note, someone clever responds: "that's like saying who cares about the right to freedom of speech if you've got nothing to say!?"
\end incoherent ramble
Indeed! this is why I prefer to use e2e encryption and possibly prefer to meet the person and scan their QRCode for verification of their address (and key exchange)