This won't be particularly helpful since I cannot remember the name of the place but there is an agency that sells flowers, balloons and lottery tickets. They also take outgoing mail but it has to be a letter or flat envelope and must have postage. No parcels. It's on the arrivals level at the far left end of the terminal.

It was a SkyClub agent who told me to go there from Terminal 2. She offered to drop my letter off in a mailbox on her way home but I didn't want to be a bother and had plenty of time to walk to T4 during a layover.