We have a hybrid Exchange setup. In our on premise Exchange console (Exchange 2016), we see an error on a room mailbox: While we don't see them in Exchange Online. And we created the mailbox in our on-prem environment. If I click the error message, 4 more errors appear but doesn't seem relevant. The room mailbox is working fine in Exchange Online and they seem to have the right attributes. Neither does it have strange characters as the last error suggests. Any idea's what I could check?

Hi @Erjen Rijnders , Have you checked this room mailbox in Exchange Online? Accoridng to my checked in my lab environment, the room mailbox created in on-premises Exchange server is synchronized to Exchange online and will be displayed in the Contact of the Exchange online admin center. Based on your error message, we could see that the externalEmailAddress error is displayed. If only the specific room mailbox met this issue, you could run the following command in the Powershell connected to Exchange online. Then compare whether this room mailbox is set differently from other room mailboxes. Get-MailUser -Identity <> | fl In addition, if only this specific room mailbox met this issue. If the room mailbox does not contain too many meetings, I recommend you to create a new room mailbox. ---------- If the response is helpful, please click "Accept Answer" and upvote it. Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

That's very helpful, thanks a lot!

Multiple errors in onprem Exchange console on Exchange Oline mailbox (hybrid scenario)

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Lucas Liu-MSFT 6,161 Reputation points

2021-02-26T09:53:31.343+00:00
Hi @Erjen Rijnders ,
Have you checked this room mailbox in Exchange Online?
Accoridng to my checked in my lab environment, the room mailbox created in on-premises Exchange server is synchronized to Exchange online and will be displayed in the Contact of the Exchange online admin center. Based on your error message, we could see that the externalEmailAddress error is displayed.

If only the specific room mailbox met this issue, you could run the following command in the Powershell connected to Exchange online. Then compare whether this room mailbox is set differently from other room mailboxes.

Get-MailUser -Identity <> | fl

In addition, if only this specific room mailbox met this issue. If the room mailbox does not contain too many meetings, I recommend you to create a new room mailbox.

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Lucas Liu-MSFT 6,161 Reputation points

2021-03-03T08:00:57.97+00:00

Hi @Erjen Rijnders ,
Do suggestions above help? If the issue has been resolved, please click “Accept as answer” to mark helpful reply as an answer, this will make answer searching in the forum easier and be beneficial to other community members as well.

Thanks for your understanding.

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Lucas Liu-MSFT 6,161 Reputation points

2021-03-09T10:28:17.15+00:00

Hi @Erjen Rijnders ,
I am writing here to confirm with you how thing going now? If the above suggestion helps, please click “Accept as answer” to mark helpful reply as an answer.Your action would be helpful to other users who encounter the same issue and read this thread.
Thanks for your understanding.

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Erjen Rijnders 61 Reputation points

2021-03-11T13:34:18.993+00:00

Hi LucasLiu,

Unfortunately that does not help, because the problem lies in the onprem exchange server, not in online.
We will probably delete and re-create the room mailbox, because it's probably more effort finding the problem.

Lucas Liu-MSFT 6,161 Reputation points

2021-03-12T10:15:33.873+00:00

Hi @Erjen Rijnders ,
If only one room mailbox encountered this issue.Rebuilding a room mailbox is indeed the fastest solution.

In addition, grant the "Full Access" permission of the resource mailbox to another mailbox.Then login to resource mailbox in outlook client. You could following the method in the link below to backup the meetings in room mailbox.
For more information: How to import and export calendar in resource mailbox?

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Multiple errors in onprem Exchange console on Exchange Oline mailbox (hybrid scenario)

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