Originally Posted by
dakaix
In T2 Departures the postbox is on the upper level opposite Giraffe and WHSmith at the far left hand side as you exit Security.
In T3 your options are the post office (if it's open), or otherwise immediately after Security - before the duty free.
The post box in T5 Departures is adjacent to the large Travelex desk, just before Fortnum & Mason and Galleries South. Theres also one behind the checkin desks but I cannot for the life of me remember where!
I walked past the one in the check in area in T5 the other day. It is up against the wall which runs across behind the desks. I think there is one near the traveled run VAT refund desks.