Forfront TMG denies access to FTP user if " Require all users to authenticate" option is selected for Internal network for web proxy client connection

Dear Sir,

We have installed MS forefront TMG 2010 SP2 on Windows server 2008 R2 OS. We have selected " Require all users to authenticate" option for Internal network for web proxy client connection. This rule allows only athenticated (domain users) to access internet through Forefront TMG and this is what we want. 

But this rule denies access to FTP users which we do not want. When we deselect the " Require all users to authenticate" option for Internal network, FTP users get access.   

Appreciate if anyone help us to resolve this.        

Thank you