The digital equivalent of the envelope is encryption - the carrier is allowed to see the outside but not the inside. The function of encryption is lack of comprehension - the carrier still knows the exact encrypted bytes of your mail, but cannot comprehend it. We generally accept that this is already a useful proxy for privacy.
Imagine a mail protocol that somehow lets you encrypt everything but the part necessary for spam identification. The carrier is able to apply some algorithm to determine whether it's spam, but no other information. I'll assume everyone thinks this sounds good (though impossible). Because encryption is just lack of comprehension, an algorithm that is only capable of parsing an "is spam" feature from email has the same level of privacy as this hypothetical. But an algorithm that is capable of parsing "your credit card payment is overdue" is able to comprehend something else, which is a different level of privacy.
(Of course, encryption is better than "They're not running anything that can parse this," because it says "Nobody can parse this.")