Sorry that I cannot give you an answer. I am having the same problem. I have the correct exchange settings. I even turn off autodiscovery and set the external and internal servers to the same settings. Mail partially works, but it will "forget" older email items when they are still in an inbox. This is NOT an issue with iOS, so I think there is a bug in the Mail program. I am still searching for an answer as well. Right now I am resorting to exchange webmail on my mac. This seems to be only a mail problem. My calendar and tasks sync fine.