[joropito]

Reading the code (meta_api.cpp / emsg.cpp / CEvent.cpp) when event it's ready to be sent to client/s (after messageEnd), amxx calls every registered forwards.

So the answer is, its called when event it's ready to be sent.

C_MessageEnd_Post it's registered as post as it's registered using at pfnGetEngineFunctions_Post call.

In fact, if I'm right, you can't block the event.