M-Zed-5 was an outpost in Klingon space.

Korris claimed that he booked accommodations on the freighter Batris with his two companions for this outpost in 2364. In his fabrication, Korris claimed that in the passage to this outpost they were attacked by a Ferengi cruiser. (TNG: "Heart of Glory")

This outpost was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 63) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Outpost MZ-5 was located in the Beta Quadrant.