Assume that you have a hybrid deployment of on-premises Exchange Server and Exchange Online in Office 365. You convert a remote user mailbox in Exchange Online to a mail user in Exchange Online. In this scenario, you can no longer sync that mail user to the cloud if that user's proxy addresses are set to use non-verified domains.
This problem occurs if the value of the msExchRemoteRecipientType attribute was incorrectly set to 8 when the remote mailbox was disabled. Non-verified domains aren't synced when the value of the attribute is 8. The value of the attribute for mail users should be Null.
Warning This procedure requires Active Directory Service Interfaces Editor (ADSI Edit). Using ADSI Edit incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems that result from the incorrect use of ADSI Edit can be resolved. Use ADSI Edit at your own risk.
To resolve this problem, use ADSI Edit to clear the value of the msExchRemoteRecipientType attribute for the user. To do this, follow these steps:
Start ADSI Edit, and then connect to the default naming context.
Expand the domain, and then expand the organizational unit (OU) that contains the user object.
Right-click the user object, and then click Properties.
In Attribute Editor, locate and then select the msExchRemoteRecipientType attribute.
Click Edit, delete the value, and then click OK.
Wait for directory synchronization to run, or manually force synchronization.
Microsoft Exchange Online, Exchange Server 2016 Enterprise Edition, Exchange Server 2016 Standard Edition, Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard, Microsoft Exchange Server 2010 Enterprise, Microsoft Exchange Server 2010 Standard