Active Directory Cleanup
If your on premises environment is AD only, and does not have an Exchange server; you will need to install the Exchange Server 2013 Schema Extension before installing DirSync.
AD Health Check Tools
IDFix – This tool looks for potential conflicts in usernames between the O365 and on premises environment and other settings that might conflict.
ADModify.net – Can be used to make batch changes to AD principals such as email addresses or usernames
Office 365 On-Ramp – This tool asseses your on premises environment to see if its ready for DirSync.
Before installing DirSync your should ensure that there ar no NULL UPN suffix values for any AD objects.
We need to add the external domain that we’re using as a UPN suffix in AD.