Abstract: A door/window/mailbox alarm system comprising a switch, a transmitter unit, a receiver unit and a warning means. A user is reminded to check and close an opened door/window/mailbox by the warning means on the receiver unit which is located away from the door/window/mailbox and in a close proximity to the user's active area such that the user could see or hear a warning signal from the warning means easily. The transmitter unit comprises a timer device that enables the transmitter unit to send out a preset signal every “x” amount of seconds to the receiver saying the door/window/mailbox is still open and serves as a reminder.