Related Clues:

• Validation for a letter — tread heavily
• Eradicates, with "out"
• Philatelists collect them
• Brand of squash
• They're collected in passports
• Passport feature
• Email doesn't require one
• ___ of approval
• Postal purchases
• They get stuck in corners
• Postage buy
• Amtrak bullet train
• Notary need
• Appropriate star of "The Collector"
• It's often stuck in the corner