Abstract: Device for automatically filling a container for collecting flat objects, such as mail envelopes, including a receiving location where the envelopes are delivered one by one, preferably in a vertical position and stood up on their edge, from a conveyor, in particular with driving belts pinching the envelopes between them until the moment when they are released into the location in order progressively to form a stack, this location including at least one and preferably two opposite parallel jogging edges extending perpendicularly to the planes of the successive envelopes in the stack formed between these edges, and a horizontal bearing sole, the first envelope in the stack furthermore being pressed against a moving stop which moves in a horizontal direction as the stack is formed, wherein the bearing sole consists of two retractable flaps placed facing one another in the same horizontal plane in order to support the stack of envelopes, these flaps being mounted so that they can pivot about two parallel axles a