When you run the Microsoft Azure Active Directory Sync tool to synchronize your on-premises directory to Azure Active Directory (Azure AD), you get the following error message in an email message that's sent by MSOnlineServicesTeam@MicrosoftOnline.com:
Error 012: Unable to update this object in Microsoft Online Services because the proxy address associated with this object in the local Active Directory is already associated with another object. Fix this in your local Active Directory.
Note Azure AD provides identity management and access control capabilities for cloud services such as Office 365, Microsoft Azure, and Microsoft Intune.
This issue occurs if one or more of the following conditions are true:
An object in the on-premises Active Directory has an SMTP address that's the same as the SMTP address of the object that's reporting the problem.
An object in the on-premises Active Directory has a mail attribute that's identical to the object that's reporting the problem.
An object already exists in Azure AD that has the same SMTP address or mail attribute as the object in the on-premises Active Directory.
To resolve this issue, use one of the following methods:
Method 1: Locate the object in the on-premises Active Directory
To locate on-premises Active Directory objects that use an SMTP address or mail attribute that's identical to the object that's reporting the problem, run an LDAP query in the on-premises Active Directory.
For example, if the problem proxy addresses is firstname.lastname@example.org, the LDAP query will resemble the following:
If multiple results are returned, you can resolve the results so that only one object has the proxy address and mail attribute with the problem email address.
Method 2: Locate the object in Azure AD
If the object already exists in Azure AD and has the same SMTP address or mail attribute as the object in the on-premises directory, you can locate and then remove the conflicting object or email address from Azure AD.
By default, directory synchronization runs every 3 hours. To minimize the effect of the suggested fix on the affected user, you can force directory synchronization to run immediately. For more info about how to do this, go the following Microsoft website: