The cloud user was originally created as byron@OnPremDomain.com and renamed to be byron@CloudDom.onmicrosoft.com. When this was done, the original address (byron@OnPremDomain.com) was kept as a proxy address. You could view both addresses when using Get-MsolUser. This address caused a synchronization error for an on-premises user named byron@OnPremDomain.com.
To resolve this error, I need to remove byron@OnPremDomain.com from the list of proxy addresses. However, you can't do this with Set-MsolUser. The mechanism for managing proxy addresses in Office 365 is Set-Mailbox. But, without a license, there is no mailbox for the user account.
The solution is to add a license temporarily:
- Add a license for byron@CloudDom.onmicrosoft.com which creates a mailbox.
- Use Set-Mailbox -EmailAddresses to remove the incorrect proxy address.
- Verify Get-MsolUser shows only the correct proxyaddresses.