Can the Return Address Label Be on the Flap of the Envelope When Mailing?
The United States Postal Service requests that the address and the return address be listed on the same side of the envelope, with the return address in the upper left hand corner of the envelope. Failing to put the return address in the correct location can cause the item to be delayed or lost in the event it needs to be returned to the sender.
To avoid delays when mailing an envelope or to ensure that it is returned to the proper address, the USPS website states to use the correct address and abbreviations for both addresses. While the post office will most likely still be able to return a letter to the sender if the return address is improperly placed on the flap, it is not guaranteed, and therefore doing so is not recommended.