This reading of mail was NOT restricted to "known suspects" as collaborators with either Germany or Japan. It applied to ALL such mail. And during WW II, the Postal Service was the equivalent of the Internet today.
Facts are useful things. But they have to appear in public to be useful.
Congressman Billybob
Latest article: "Rep. Taylor's Office Caught in an Obvious Lie in Attempt to Smear John Armor"
Or the history of electronic intercepts, for that matter.
Was that practice (opening the mail) challenged in court?