All postage stamps issued by the United States since 1860 are valid for postage, with exceptions that are covered in the Domestic Mail Manual (DMM) . The following types of stamps are valid for postage:

• Forever Stamps: These stamps can be used to mail a one-ounce letter regardless of when the stamps are purchased or used and no matter how prices may change in the future.

• Postage Stamps with a Denomination: These stamps have a specific denomination printed on them and can be used to mail letters or packages up to the weight indicated by the denomination.

• Nonprofit and Presort Stamps: These stamps are not listed in the valid/invalid paragraphs, but the USPS considers them to be valid for postage.

Stamps that are mutilated, defaced, cut from stamped envelopes, aerograms or stamped cards, "covered or coated in such a manner that canceling or defacing marks cannot be printed onto the stamps," or overprinted with an unauthorized design, message, or other marking are invalid for postal use. If you have any doubts about the validity of a stamp, you can check the Domestic Mail Manual or contact your local post office for assistance.