Interpreting the tenuous basis for privacy protection drawn out of the 4th Amendment is certainly challenging in the post-network world. Case law on protections, or lack thereof, for different forms on online communications is all over the map.
According to The Volokh Conspiracy, the 11th Circuit has made a very narrow ruling in a recent case that will no doubt constrict any such protections further in that jurisdiction. Orin Kerr has some excellent analysis supporting his contention that this ruling is incorrect.
The ruling is in response to a defendant’s claim, after dismissal of the original case, that the acquisition of his emails from his ISP via subpoena violated his 4th Amendment rights.
I encourage you to read Kerr’s argument but in short, the court ruled that once a copy of your email stored with an intermediate third party is delivered to the end recipient, all copies become fair game. Kerr contends a better ruling would require that different copies (ISP’s intermediate copies, intended recipients’ copies) be treated differently.