I completed a migration from Exchange 2007 to 2013. After decommissioning the 2007 server and moving log files to another location. I had a couple users receive an error on their iPhones "unable to verify account information". Under IIS --> Authentication, I enabled Windows Authentication on the Default and Back End websites and restarted IIS. That seemed to work, however if I add myself to someone else's iPhone the same error appears again "unable to verify account information". What I thought was the solution only seems to be just temporary. I have checked the log files under inetpub and I don't see any access denied errors for those specific users. I was curious if anyone else has experienced this and might have found a solution? If you need me to provide more information let me know.
Thanks.