If it is so, it is done with codes that I cannot understand easily. (I was always wondering how the sorting machine knows - but this would be a simple question for a postman!)

I recently received a credit card from CITI in an unmark white envelope. It has a P.O. BOX return address in St. Louis and some printed code under my address (062, combination of letters and numbers and the ZIP code). In the postage stamp area, there is just a rectangle saying PRESORTED FIRST-CLASS MAIL U.S. POSTAGE PAID C.C. S.P.
Perhaps the C.C. means Credit Card, which would disqualified it from forwarding service.