Related Clues:

• It may be junk
• 'What ___!' (possible response to 20-, 29-, 46- and 56-Across)
• It often contains lots of letters
• It might be four letters
• Make __ (settle)
• Vacationer's accumulation
• Drop in a letter box
• Postal deliveries
• Letters and such
• Drop in a letterbox
• Letters, collectively
• ''Let's Make __''
• Drop in a slot
• It's not delivered on Sundays
• Postal sackful