I think the confusion you and I share is that the topic of the precedent is mail that is physically stored on a 3rd party's server. The transport mechanisms are irrelevant.

Now, what if you used a service like gmail, but made sure that you deleted all your mail before they aged to 180 days. What if the mails are not actually deleted from the underlying storage, but just not presented to you. If the mails still "exist" could they be used against you? Would google give them up? Was there an expectation of actual deletion?