unable to synchronize mail on phone

Hi all, 

we have a problem only with one user:

This user can send and receive mail via phone only via wifi, when he try to use the mobile connection, the synchronization is not work.

we have change phone, and we have the same problem. We also tried to insert another email address in the phone and it works fine. Any suggestions? the user have the same configuration of all the others, and they are working correctly.