I was reading through this AskMe today, and it started me thinking. Emailing yourself five or ten years in the future isn’t a difficult problem (in fact, that thread includes several solutions). But what would be the most reliable way to get a message to yourself in, say, 50 years? Not necessarily an email, and inexpensiveness and plausibility are important (becoming President and renaming the country “Hello Future Me” won’t work).
Would Doc Brown’s method of sending a letter from 1888 1885 to Marty standing in the road (not even an address) in 1955 even work back then? I doubt it. How could it be done today?