Many fileservers are themselves behind firewalls, normally because they're hosted on university or corporate connections. In order to get around the problems associated with this there is a system known as "reverse dcc" or "alternative dcc method" or other such designation. When a server uses this method there will be an instruction in the server's advertisement that tells you to use the "/dccserver" command.
Make sure you follow those instructions or there is no way a send can get through.
John.
