Look up 'mail cover' ~ the laws are written to deal with INTENT, not the fact but strictly intent since, after all, a postal employee ~ a gub'mnt agent ~ will touch that letter and read the information on the cover and deliver it, send it back, or whatever.
The cover information is an open book to be read by all ~ and the call, email and other telecommunications bric a brac packed in and around the message, whatever it is, is also equally open BUT, as open as it is the Congress wrote laws to regulate mail covers and electronic meta data.
I find it absolutely amazing you guys read these articles and skip on by the fact that today the USPS actually makes an electronic image of the front and back of each and every mail piece that requires initial sortation ~ in effect, they do a mail cover ~ and all without a warrant anybody knows about. Still, they do it confident they'll never be called to account ~ do you imagine there is a broad spectrum FISA court warrant in place?
Frankly, I have no idea ~ but it is still amazing people don't seem to care about these covers but once, not long ago, they wanted to bust Nixon's chops over them.
We quit sending things by mail and started sending everything via some method on the internet. Guess the next step is to deliver everything in person.