'mail slot' definitions:

Definition of 'mail slot'

(from WordNet)
noun
A slot (usually in a door) through which mail can be delivered

Definition of 'mail slot'

From: GCIDE
  • mail slot \mail slot\ n. A usually horizontal slot in a door through which mail can be delivered. It often has a hinged cover to keep the opening cloised when not in use. [WordNet 1.5 +PJC]