Related Clues:

• Unopened
• Tamper-resistant
• Irrefutable
• Like most mail
• Impenetrable
• Signed and delivered go-between
• Ready for mailing
• Shut up
• Unassailable, as an alibi
• Part of S.W.A.K.
• Sealed
• Perfect, as an alibi
• Airtight
• Closed up
• Made airtight