I cannot see Multiplay hosted servers when my firewall is up.
I have set all the standard ports and can see a shed load of other servers, but not, for example freeforall.
I have noticed you use non-standard port ranges, but so does eauk and I have opened the game ports associated with their servers and can connect (I could see them before but not connect).
Anyhow, I hope that makes sense, what I am trying to say is that I could see and connect to other servers, could see and not connect to other servers (until I made the gameport adjustments), but not see any of your servers.
I noticed this last night when our clan server went down so we visited freeforall. The two people with a firewall couldn't even see the server, let alone connect, but could see and connect when we dropped our firewall (something I hate doing because I have to reenter all the ports into my hardware firewall).
Does this have anything to do with being behind a NAT router?