This is a composite character. In the centre is a pictograph of a messenger pigeon. Then, the noun messenger pigeon is surrounded by the glyphs which mark the beginning and the end of an area. A
space in which a messenger pigeon was confined would be a cage.
Illustrative Text Reference:
Harappa: Seal: H-142 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 200: Collections in India: Suomalainen Tiedeakatemia.
The fact that the pigeon is caged tells us that a message is to be sent, as opposed to received.