NFS Local ok, but no remote connections

[JoopSiroop]

My 2008 R2 NFS server can not be reached from outside the box.
Using the mount command in aa local command shell, I can connect to an exported share.
But from another pc, I can not get a connection. a linux box reports mount system call failed, a windows 7 box says Network Error 53.
I disable windows firewall, removed the file server role completely and reinstalled the file server role.
I am very sure there are no firewall/anti-virus or other software components running.

Any ideas on how to solve this?

[Hadassah]

I would like to tell you that the Error 53 comes if the same resolutions fails for some particular computer name. Other than this the Error 53 ca n also happen if there is some issue for establishing a NetBIOS session.