Copyright infringements is not, even in principle, about seizure or possessory interest, so there is no relation between your question and the quote it is supposedly based o .
Neither is search and seizure about a possessory interest. It's about privacy and security. Even in the eighteenth century it was about "persons, papers, and effects." It was always an information security question.
A seizure is defined, basically, as something that harms a liberty (if a person is the subject) or possessory (if the subject is not a person) interest; without that there is no seizure, reasonable or unreasonable.
Likewise a search is something that harms a privacy interest.
Thank you. I'm saying the above somewhat tongue-in-cheek, because my gut feeling is that some aspect of interest has been violated in the case of the email owner or related parties. Of course, "gut feeling" is not a legal principle. So I'll go back to being Definitely Not A Lawyer.