Google asserts that, in principle, if you entrust your personal messages to a third party, you can't expect that the third party won't touch any of that information:
Just as a sender of a letter to a business colleague cannot be surprised that the recipients assistant opens the letter, people who use web-based email today cannot be surprised if their communications are processed by the recipients ECS provider in the course of delivery. Indeed, a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties. Smith v. Maryland, 442 U.S. 735, 743-44 (1979). In particular, the Court noted
that persons communicating through a service provided by an intermediary (in the Smith case, a telephone call routed through a telephone company) must necessarily expect that the communication will be subject to the intermediarys systems. For example, the Court explained that in using the telephone, a person voluntarily convey numerical information to the telephone company and expose that information to its equipment in the ordinary course of business.
Google also argued that restricting how email providers are permitted to process the data they receive could "criminalize" features like spam filtering and inbox searches:
Last, Plaintiffs claims should be rejected because they would lead to anomalous results with far-ranging consequences beyond the allegations in the Complaint. Plaintiffs theorythat any scanning of email content by ECS providers is illegalwould effectively criminalize routine practices that are an everyday aspect of using email. Indeed, Plaintiffs effort to carve out spam filtering and virus detection from their claims underscores the fact that their theory of liability would otherwise encompass these common services that email users depend on.