When I've been away from home for a month or more, I've stopped the build-up of mail by resting a board at about 60deg under the letterbox. Letters just slide down it further into the hallway. Doormat in the right place stops the board sliding down and effectively jamming the door. 60deg is good enough so that opening the door pushes the board into the vertical position. It's a bit fiddly setting the board up with one hand round the door as you close it but for a month long trip it's worth it and if you were to do it every day I dare say you'd get quicker at it.
Nick