Does this still appy when logginh onto the remote FTP site with a user name/password rather than using windows authentication?
Basic authentication is set up by default when using FTP, However; you can use digest authentication where it will use the AD credentials and automatically send them over encrypted. I believe that it auto enters those credentials and skips the need to use a username / password (could be way off here, not 100% as Ive never used it).
So it sounds like w/e ftp your trying to access is using basic authentication. I'd get in touch with the tech your trying to access and see what settings they have set up. You can use a lot of different things for security, IP, Trusted domains, etc.