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 jsmith@contoso.com, the LDAP query will resemble the following:
(|(proxyAddresses=smtp:JSmith@contoso.com)(mail=JSmith@contoso.com))
This LDAP query looks for all objects in Active Directory that have a proxy address or mail attribute value that contains JSmith@contoso.com.
You can use a tool such as Active Directory Users and Computers to run an LDAP query. For more info about how to use such a tool, go to the following Microsoft TechNet website:
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 Windows Azure AD
If the object already exists in Windows Azure AD and has the same SMTP address or mail attribute as the object in the on-premises Active Directory, you can locate and then remove the conflicting object or email address from your organizational account.