[nickodell]

  It's not like the subpoena is a new legal instrument

True, but there isn't an 18th century analogue to a company that has all of your personal correspondence.

[mpyne]

There were newspapers, which people would post open letters in. I seem to remember a Reddit post about some gentleman calling another one a "scoundrel" and a "coward" and challenging him to a duel.

Later (in the 19th century), there was Western Union...

[danielweber]

I like this point, but I have some questions.

1. What information would the newspaper keep that isn't published? Would they file all the envelopes of regular correspondence?

2. How deep and old would Western Union's records be?

[mpyne]

1. The envelopes, probably not. They would probably at least keep a log of correspondence received and sent, accounts payable and receivable, payment receipts, etc. If they have stories that make factual claims then they'd want to retain the journal/notes that went into each story for some nominal amount of time as well, if only to defend their name later.

2. Don't know, but the easy answer is "As long as the government required it to be". Even the NSA gets rids of their data after 5 years (or so they say).

[rayiner]

In the 18th century as now you could have chosen to give all your personal correspondence over to some company.

[nickodell]

But why would you? There's no purpose to having a company store all of your letters.

Using GMail gives me a free email account with lots of storage, search, and spam filters.