I have about 12 clients that have iPhones with our exchange account and 8 of them sync and push fine. The other four get cannot connect to server. I have done an exchange sync test that works fine. I can also log in via OWA to their account fine. I have checked their software updates and they are fine. One user has an iPad, Mac, and iPhone all say the same thing "cannot connect to server" .
All settings are correct for these four users. I am thinking it is something in the AD security?