error when using domain mapping

Im using domain mapping and I get this error at the bottom of the website Im trying to map:

Warning: Unknown: open(/tmp/sess_6fb6a75fe60471a9657b60b9c8efd7a5, O_RDWR) failed: Permission denied (13) in Unknown on line 0

Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0

Any help would be most appreciated.
Regards
Dave