On Mon, 20 Apr 2015 13:01:15 +0100, Chris J Dixon
wrote:

Davidm wrote:

You can get change over reed switches (google it). Wire it to use the
contact pair that closes when the magnet on the flap moves away from
the switch.

That could provide a signal of the right sense.

Does the OP need a circuit which latches, for the rare occasion
when the postie doesn't leave the mail stuck in the flap, with a
gale howling through the gap? ;-)

Well, that doesn't happen, but it would be nice to have the alarm
alert for 10 seconds before switching off. For starters, it's enough
to just have the gadget sound once. For just a letter, it wouldn't
sound for very long, but I usually get either no mail or a bundle of
up to four items and it takes longer to push those through.

MM