If you want to ensure they can’t read your e-mail, then you have to keep the keys in your possession. If you use their encrypt-at-rest solution, then they nece

If you want to ensure they can’t read your e-mail, then you have to keep the keys in your possession. If you use their encrypt-at-rest solution, then they necessarily have the keys. But if they can’t read the e-mail, then they can’t process it for you and act as your service provider. Otherwise, how would they know what to do with the black box you have given to them? If you’re going to use an external service provider, then at some point you have to trust them, at least to a degree.