Should call SingPost & inform them abt the postmen doing that...... there was a period when this postman was really "forgetful" or what, he left the letterbox door behind open..... my hubby caught that few times in the evening when he was back and collecting letters.....that week, he called SingPost and they quickly sent somebody at late evening to lock it cos my hubby warned them if any Important letters are lost, they've to face legal suits...
Maybe u shd ask ur hubby to call singpost to complain about the 'lock' ....