> I was under the impression (from some discussion about electronic search-and-seizure laws) that mail, once delivered to the recipient, is no longer considered your property.

While true, owning a piece of mail does not imply that you own the relevant copyright.

Consider a book that you own. You own that copy. You can destroy it, you can sell/give/lend/show it to someone else, you can keep it. That physical thing is yours do do with as you will.

However, you can't (legally) copy it if its copyright is still active. (Yes, fair use comes in here too.)

Physical and electronic mail is exactly the same.