Also the mail has access to this metadata.
The basis of using a service is that you entrust them with the care of your necessity. That they make it unable by policy not to look at the contents of your mail is a nice feature, but nor necessary, as we firstly rely on the good faith we put on the service provdider. To go ahead and request that the security measures be extended to data necessary to route communications, is of a pathological paranoic nature.
The mailman needs to know the address.
Mailmen who don't require your address to deliver may appear as competitors, but they will always be a shady second choice, because mailmen need to know your address.