> Your talking statutes, not the constitution
I'm talking both. The ECPA was important in that Congress avoided decades of court cases by making explicit the protections afforded electronically stored media, though they did not extend those protections far enough (which today in practice weakens protections that may have been more clearly delineated by now had the ECPA not been enacted).
Constitutional protection superseding (among other things) the fairly arbitrary 180 day requirement for a warrant set by the ECPA was clearly recognized by the Sixth Circuit in the US v Warshak second (criminal) case, stating that "The government may not compel a commercial ISP to turn over the contents of a subscriber’s emails without first obtaining a warrant based on probable cause."[1]
In both US v Warshak cases, though, the Sixth Circuit emphasized the higher protection afforded content over transactional data just for being content by the the tests established by both Katz v US and Smith v Maryland. They laid out that even the supremely terrible precedent of Smith v Maryland (which is the proud parent of allowing the government to seize "metadata" without a warrant) did not allow the government to "bootstrap" limited access to full access, including the access needed for automated processing of email contents by the email provider:
"The government also insists that ISPs regularly screen users’ e-mails for viruses, spam, and child pornography. Even assuming that this is true, however, such a process does not waive an expectation of privacy in the content of e-mails sent through the ISP, for the same reasons that the terms of service are insufficient to waive privacy expectations. The government states that ISPs “are developing technology that will enable them to scan user images” for child pornography and viruses. The government’s statement that this process involves “technology,” rather than manual, human review, suggests that it involves a computer searching for particular terms, types of images, or similar indicia of wrongdoing that would not disclose the content of the e-mail to any person at the ISP or elsewhere, aside from the recipient. But the reasonable expectation of privacy of an e-mail user goes to the content of the e-mail message. The fact that a computer scans millions of e-mails for signs of pornography or a virus does not invade an individual’s content-based privacy interest in the e-mails and has little bearing on his expectation of privacy in the content. In fact, these screening processes are analogous to the post office screening packages for evidence of drugs or explosives, which does not expose the content of written documents enclosed in the packages. The fact that such screening occurs as a general matter does not diminish the well-established reasonable
expectation of privacy that users of the mail maintain in the packages they send."[2]
I have not personally seen a good argument for differentiating between spam filtering and contextual advertising in terms of access. Regardless, this is a clear argument for automated access being immaterial to the question of an expectation of privacy of the contents of an email.
[1] http://www.ca6.uscourts.gov/opinions.pdf/10a0377p-06.pdf
[2] http://www.ca6.uscourts.gov/opinions.pdf/07a0225p-06.pdf