Removing 365 mailbox email alias in hybrid setup

Question

Our mailboxes are in 365, but we are in a hybrid setup with AD and Exchange 2016. We have one mailbox that has an alias address associated with it that we'd like to remove. This was set up in on premise Exchange before it was migrated to the cloud. In attempting to removing the alias - either via PowerShell or EAC - it errors out. It says 'An Azure Active Directory call was mmade to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Detailed error message: Unable to update the specified properties for on-premises mastered Directory Sync objects or objects currently undergoing migration.'

Well, its not undergoing migration. We do not have writeback turned on from 365 back to on premise AD. How can we remove this?

Answer 1

@Close, Andrew

Glad to see this question is resolved now. Due to an update to the forum rules, the question author cannot accept their own answer. They can only accept answers by others.

So, let me summarize the question for you, if could, you can accept this one as an answer to help us close this thread which also could be beneficial to other community members reading this thread.

For migrated mailbox, most properties need to be modified from local AD: "This user is synchronized with your local ‎Active Directory‎. Some details can be edited only through your local ‎Active Directory‎."

About the alias, we could modify it from proxyAddresses on AD account. If there exists Exchange on-premises, we can also modify this one from Exchange on-premises (Migrated mailbox is show as Office 365 on Exchange online)

This is also the reason why the hybrid environment keeps Exchange on-premises after migrated all mailboxes to Exchange online.

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Answer 2

Figured this out. You just go into the on premise AD and in the proxyaddresses attribute remove the alias address and then force a sync with 365 and that removes it from 365.