Related Clues:

• Hard-to-break plates
• Snail ___
• It may contain a lot of letters
• Four letters?
• Blue box material
• It can be registered
• Viewer ___
• See 46 Across
• Spam, say
• Use the USPS
• Vacationer's accumulation, perhaps
• Letters in a box
• E or G follower
• Bills, circulars, etc.
• Post office sackful