This is a situation we resolved for a customer who was having issues when external people were trying to send to some of their distribution groups.
Installing DirSync is not fun, easy or short. Do not make any assumptions about how long it will take to setup. I say this because I spent several wasted hours trying to use the default install and let it install SQL Express automatically and it failed with a whole host of different errors each time I uninstalled, cleaned up and tried again.
Official documentation was either out-dated or used methods which didn’t work, blogs were a mixed bag as well. This post tries to tie the various pieces of correct information together. Continue reading Cut Over Migration and DirSync with Office 365