Behind MS Proxy FTP error

Hi, I am behind my MS Proxy 2.0 (SP1) Server & have unlimited rights to all protocols (equivalent to Administrator). I can download files thru FTP & HTTP easily from the Internet. Problem is I cant access FTP Servers, I have used Cute FTP, manual Login. I can login to the FTP Server & when I type “ls” or any other command I get “FTP:Bind: Can’t assign requested address.
The damm thing works from systems having valid Internet IP but it does not work from behind the Proxy. Do I need to open ports 21 etc., If yes, how & how do I check that ?
Akash