Windows 2003 FTP from Mac problem

I've seen several posts on this issue. Some say the had been fixed,
yet did not list an explanation or solution and all attempts to contact
the original poster produced no responses:

Windows 2003 Web
FTP site (No anonymous), file permissions set fine

Access from anything windows works fine. A client with a Mac network
can get nothing Mac (OS X FTP, or any othe Mac FTP 3rd party clients)
to work. IE on their machines DOES work. WTF? Most of my research
suggests firewall issues, but why would a firewall on either end treat
IE different from Mac stuff? 2k3 web has no firewall, and I haven't
installed one...I've made sure one of their users did not have Mac OS X
Personal Firewall enabled, still only IE. They claim no network
firewall but with their tech-know, I'm not banking on that.
Help would be much appreciated,
Dominic.