Office 365 Exchange Online: Unable to Enable the UMMailbox

Customer is in the Exchange Hybrid Configuration with Office 365 and Exchange 2016 CU 11.


While enabling the UM mailbox, customer got the following error:

Unified Messaging can’t save the PIN for mailbox “”: There was an error accessing the mailbox using EWS. The requested operation failed with ResponseCode “ErrorInternalServerError”. Details : An internal server error occurred. The operation failed., Access to Active Directory failed. .


This mailbox has hide from address book checked. So, Exchange online could not enable UM Mailbox for this user.

Logged in to the Exchange 2016 ECP, unchecked the checkbox “hide from address lists” to show the user in the address book.

Then replicated the user properties to office 365 Exchange online using AADConnect.

Now Enabled UMMailbox.

Once UMMailbox was enabled, checked the checkbox hide from address lists and replicated user properties to the office 365 Exchange online using AADConnect.

Prabhat Nigam
CEO @ LAEXUG Foundation | CTO @ Golden Five Consulting |

CTO @ Golden Five | CEO at LAExUG Foundation

Prabhat is 3 times Microsoft MVP Award winner. He is MBA in Information Technology and he is working as a CTO at Golden Five Consulting which is a Microsoft Gold Partner, MSP, T1CSP, and Education partner. He helps in designing, implementing, managing and supporting solutions for private messaging cloud, mergers, a collaboration between different messaging software and other migration & deployment projects for the following technologies Office 365, Azure, AWS, Exchange, SQL, ADFS, MFA, FIM, MIM and Directory services. He has worked for all big IT giants either as an employee or contractor where he has led the Global teams. He has started his career as Technical Consultant in Exchange 5.5 with Microsoft PSS and his exchange love never stopped & continued with 2000/2003/2007/2010/2013/2016/2019/O365. At one point in time, he was the only person to support EDS customers when Microsoft had closed all the supports for 5.5 and now for all old legacy Exchange versions.
He used to blog at, manages multiple LinkedIn and Facebook Groups. He also Owns MSExchnageGuru YouTube channel where he uploads all his records technical sessions. Don’t forget to check his PowerShell scripts which are making admins life easier. Prabhat can be reached at

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