1. Plural of letterbox; a box or slot, typically mounted on a wall or door, into which mail is delivered.
Usage: British English; American English typically uses 'mailbox'
2. Plural of letterbox; a public box in which outgoing mail is deposited for collection by postal workers.
Usage: British English; American English typically uses 'mailbox'
3. In film and television, the black bars that appear at the top and bottom of the screen when displaying widescreen content on a standard screen.
Usage: technical, informal

Examples

• The postman delivered letters to all the letterboxes on the street.
• We installed new letterboxes at the entrance of our apartment building.
• She checked the letterbox every morning for important documents.
• The old red letterboxes are iconic symbols of British postal service.
• When watching the film on a standard television, letterboxes appeared on the screen.
• Several letterboxes were damaged during the storm last week.