Related Clues:

• Unopened
• Tamper-resistant
• Like most mail
• Impressed
• Part of SASE
• Signed and delivered go-between
• Ready for mailing
• Shut up
• Part of S.W.A.K.
• Airtight
• Closed up
• Made airtight
• Tamper-resistant, perhaps
• Ready for the post office
• Ready to post