Here's an idea.
Imagine you go back in time, and you ask Jefferson, Washington, Franklin and friends whether it would be OK if the government was keeping a record of who sent every letter and who they sent it to. Just to protect us, obviously.
Just imagine that for a moment.
Then tell us, with all seriousness, that you really, actually, imagine they would say 'yeah, sure, that's fine, and totally Constitutinal'.