Post all open letters here.
If you use their real name (and the place they're from, to narrow them down if their name is common,) then hopefully Google Alerts (Yes, "Google Alerts" is a thing) will have an alert for their own name set up like it is for mine, and let them know that a letter is ready for them to read.
Otherwise, if they don't have a Google Alert set up on their name, the next time they Google their own name, the open letter to them here will pop up.
Set up your own alerts on https://www.google.com/alerts and maybe an open letter to YOU will pop up there one day!
Now, write away!