\\[destination]\mailslot\\messngr is undocumented mailslot name, though it is widely used exactly for this purpose -- for sending popup messages. As I already mentioned, this is an equivalent of NET SEND command.
In this example a message is sent to the local computer, just to avoid disturbing others. As a destination, you also can choose any computer or server name on your LAN. An asterick (*) is for the primary domain. With this name chosen as a destination you will be granted an immediate attention of all your co-workers.
On target computer, TCP port 139 (NetBIOS) is assigned to receive messages.
Also the NetMessageBufferSend function (Unicode-based) can be used for sending messages on a network. If compared to this function, the mailslot approach can send messages no longer than 128 characters. Though it returns much faster than sometimes the NetMessageBufferSend does.