Related Clues:

• Characteristic mark
• It's usually stuck in a corner
• Snail-mail need
• Licked square
• Envelope stick-on
• Perforation site
• Mailing need
• If it's canceled, it's accepted
• Forever ___
• An e-mail doesn't need one
• Corner piece?
• Notary public's device
• "Forever" post office product
• Impress with a mark
• Evidence of payment of postage duty