it usually means that host didn't forward his UDP Port 4321 and so he's not reachable. If someone else can connect to him, it may mean he's reachable after a NAT Punchthrough, but you're behind a CGNat, which doesn't allow NAT Punchthrough.
ManuTOO == Mana Games == >> I don't answer Private Message, except if it's really a _Private_ topic <<