Altoona Tribune, Pennsylvania, March 19, 1926

From Wikipedia:  Post Office is a kissing game played by boys and girls at parties. It has been referred to in United States popular culture since at least the 1880s.

The group playing is divided into two groups – typically a girl group and a boy group. One group goes into another room, such as a bedroom, which is called “the post office”. To play, each person from the other group individually visits “the post office”. Once there, they get a kiss from everyone in the room. They then return to the original room.

Once everyone in the first group has taken a turn, the other group begins sending members to the first room.