Are U.S. special delivery stamps valid for postage?

The last four U.S. special delivery stamps were issued from 1954 to 1971.

U.S. Special Delivery (postal service) Facts for Kids

Special Delivery Stamps - Guide to Value, Marks, History

Special Delivery 1894 10c Blue Plate Proof on Stamp Paper (E4P5),, Stamps Online: The Michael Bakwin Collection, Books & Manuscripts

E6 - 1902 10c Special Delivery - DL watermark, ultra marine - Mystic Stamp Company

Daniel F. Kelleher Auctions, LLC Sale - 778 Page 19

U.S. SPECIAL DELIVERY Mint & Used Stamps

1969 45¢ Arrows. Issue Date: November 21, 1969. At first, the service was available from 7 a.m. to midnight. The following year, the hours changed

Set of 4 U.S. #E22 1969 45¢ Special Delivery postage stamps, good to MNH

USPS 2021 Happy Birthday Forever First Class Postage Stamps,Celebration, Invitations, Weddings, Birthdays, Anniversary, Holiday, Engagement, New Job, Retirement, Parties (1 Sheet, 20 Stamps) : Office Products

U.S. STAMPS & POSTAL HISTORY

Jay Smith & Associates: United States: Stamps: Special Delivery

Postage stamps and postal history of the United States - Wikipedia

Illegal Use of Postage

Sold at Auction: US Certified Mail, Special delivery, Air Post Mail stamps, Used, Unhinged

E22 - 1969 45c Special Delivery - Mystic Stamp Company