AlpineBob on page 491
Once a time traveler knows where you posted a letter from, he can go back and be there when you mailed it.
Well, it's a start, but a postmark only says which post office processed the mail, not which of possibly thousands of post boxes in that area it was dropped in.
But a time traveler has an edge - he can stake out every post box in the area, one after the other, like a very boring groundhog day. Go back a day, watch a new box. Repeat as needed. It might take a few years of the same day, but it'd work eventually...
Probably would freak out the postmen though, to find another trench-coated emo dude at every stop they make...