Starfleet Command Internal Mail System was a Starfleet mail delivery agency, delivering mail between Starfleet Command personnel.
In 2256, when Michael Burnham received Philippa Georgiou's last will and testament, the name of this organization was emblazoned on a small sign inside a container holding Georgiou's telescope. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")