Kerr frames the question the case raises like this:
Assuming that e-mail account-holders generally have Fourth Amendment rights in the contents of their e-mails, as courts have so far held, when does a person’s Fourth Amendment rights in copies of sent e-mails lose Fourth Amendment protection?
The court held that there was no reasonable expectation of privacy. The answer to the question in this case turned on the similarities of email to physical letters -- and analogizing the reasonable expectations of privacy that parties have in physical letters:
A sender of an e-mail loses his or her reasonable expectation of privacy in an e-mail that has actually reached the intended recipient. Guest v. Leis, 255 F.3d 325, 333 (6th Cir. 2001); see also United States v. Lifshitz, 369 F.3d 173, 190 (2d Cir. 2004) (An individual may not “enjoy  an expectation of privacy in transmissions over the Internet or e-mail that have already arrived at the recipient”). “The e-mailer would be analogous to a letter-writer, whose `expectation of privacy ordinarily terminates upon delivery’ of the letter.” Guest, 255 F.3d at 333 (quoting United States v. King, 55 F.3d 1193, 1196 (6th Cir. 1995). In Warshak v. United States, the United States Court of Appeals for the Sixth Circuit analogized a search and seizure of e-mails through an ISP to the interception of a letter at a post office. 631 F.3d 266, 286 (6th Cir. 2010). Here, transmission of all the e-mails and texts was complete before seizure, that is, the e-mails had already arrived at the intended recipient account. Any reasonable expectation of privacy Mr. Lustyik and Mr. Thaler may have had in their e-mails ended when Mr. Taylor received the messages in his and AISC’s e-mail accounts.Kerr asks whether this reasoning indicates that the sender's expectation of privacy terminates when the email reaches the email account's Internet service provider (ISP), rather than when it is accessed by the recipient. He concludes that this is the conclusion that the court's reasoning supports.
While some of the opinion's language supports this reading ("Any reasonable expectation of privacy Mr. Lustyik and Mr. Thaler may have had in their e-mails ended when Mr. Taylor received the messages in his and AISC's e-mail accounts"), I am inclined to read the opinion as indicating that some sort of access by the recipient is required before a reasonable expectation of privacy is lost.
I arrive at this interpretation by considering the third- and second-to-last sentences of the above quote, where the court seems to be distinguishing the downloading in this case from a search of an ISP. The use of the word "here" in the second-to-last sentence signals to me that the court is differentiating this case from the Warshak case it summarizes in the preceding sentence, thereby indicating that this case is different from the government's downloading of emails from an ISP.
My reading may also be supported by the word "received" in the last sentence -- as this term could be read to indicate an act of receiving and reading an email, rather than the simple transmission of the email to an account's server. I admit, however, that this last point involves a strained interpretation of the word "received" (as my definition of "received" includes the word itself in conjunction with a different word).
But Kerr is correct to point out that the opinion seems to support the conclusion that there is no reasonable expectation of privacy in an email that is stored in the email account's server, as opposed to one opened by the recipient. The court's distinguishing language aside, its phrasing ("transmission of the e-mails and texts was complete" and especially the phrasing of the final sentence) lends support to Kerr's interpretation.
And if the court did indeed conclude that there is no reasonable expectation of privacy in email in an account's server, that would probably be a stretch for the physical mail analogy. While downloading an unread email that has been transferred to an email account may not be perfectly analogized to seizing mail at the post office, it might be more akin to taking unread letters from a recipient's mailbox. And that would probably be a stage of delivery where the sender still has some reasonable expectation of privacy in the letter.