When attempting to merge users from a source on premise Active Directory environment with existing users in an Office 365 environment that were initially created with Microsoft DirSync, the merge will fail and the following error will be seen in the Directory Migration Agent (DMA) log:
1/29/2015 10:00:41 AM 2308 Error 0x80131501. DirSync Users are not allowed to update.
Either of the following actions can be taken to resolve this issue:
In the Migration Manager for Active Directory (Microsoft Office 365) console, on the Mapping tab for the collection, delete all rules where Target Attribute is set to EmailAddresses. Do this for all collections and/or migration pairs that are affected by the problem.