Explanation/Reference:
Explanation:
One of the first steps you should take before installing DirSync is to look at the directory that you have on- premises and make sure it's healthy and ready to synchronize to Azure Active Directory.
You need to check Active Directory remediation.
DirSync has certain requirements on attributes in the directory, and aligning the attribute values with the DirSync requirements is commonly known as Active Directory remediation. To help with Active Directory remediation, you should use the IdFix tool, which reviews the directory and performs interactive Active Directory remediation. This tool checks for and helps you correct any invalid data and duplicate data in directory attributes, including user PrincipalName (UPN), mailNickName, proxyAddress, sAMAccountName, targetAddress, and others. The IDFix tool also provides assistance for migrating from a non-routable UPN (such as "domain.local," for example) to an Internet routable domain name, because using an Internet-routable domain is one of the requirements for Azure Active Directory. Be sure to run the IdFix tool from within your network, so that it has access to the domain controllers.
References: https://blogs.office.com/2014/04/15/synchronizing-your-directory-with-office-365-is-easy/