Historical clue formulations

• Like an envelope that's ready to be sent — 2025-12-08
• Shut up — 2023-10-11
• Closed up — 2022-11-25
• Partner of "signed" and "delivered" in a Stevie Wonder hit — 2019-06-17
• Like an envelope that's ready to be mailed — 2013-12-30
• Shut up — 2011-11-23
• Like diplomatic pouches — 2007-01-21
• Ready for mailing, as an envelope — 2005-11-22
• Made airtight — 2004-09-27
• Part of S.W.A.K. — 2003-03-18
• Shut up — 2002-08-11
• Signed and delivered go-between — 2001-10-22
• S.W.A.K. part — 2000-09-27
• Like most mail — 1998-07-30
• Tamper-resistant — 1998-03-01
• Zipped — 1994-01-14
• Used wax, perhaps — 1994-01-12