Interestingly, postmasters can act as mail proxies.

A stamp collector who wants a particular cancellation stamp can send a letter to the postmaster containing a letter and a note: "Please postmark and deliver".

I have used this in the past to play small tricks on friends, and to send them mail from far-away places.

Per the article, the entire course of your letter is tracked, so this should never be used when actual privacy is desired.