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Question

Shared mailbox backup is failing.

  • June 20, 2026
  • 2 comments
  • 12 views

VB365 Exchange Online backup is failing for 1 mailbox out of 600. Initially received EWS ErrorMailboxStoreUnavailable (“The mailbox database is temporarily unavailable. Cannot open mailbox”). After reruns, 2 affected mailboxes succeeded but 1 mailbox continues to fail with HTTP 403 Forbidden.

The O365 team confirmed the user account is disabled, the Exchange license has been removed and the mailbox was converted to a shared mailbox.

Has anyone seen this behavior with recently converted shared mailboxes? Can VB365 back up an unlicensed shared mailbox or are there specific mailbox attributes that should be verified? VB0365 Exchange Online backup is failing for 1 mailbox out of 600. Initially received EWS ErrorMailboxStoreUnavailable (“The mailbox database is temporarily unavailable. Cannot open mailbox”). After reruns, 2 affected mailboxes succeeded, but 1 mailbox continues to fail with “Failed to access mailbox, the request failed with  HTTP 403 Forbidden.

The O365 team confirmed the user account is disabled, the Exchange license has been removed and the mailbox was converted to a shared mailbox.

Has anyone seen this behavior with recently converted shared mailboxes? Can VB365 back up an unlicensed shared mailbox or are there specific mailbox attributes that should be verified?

2 comments

MicoolPaul
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Hi,

 

This is a workload undergoing heavy change. Microsoft has deprecated EWS and is in the process of retiring it. This error happens because Microsoft changed a behaviour regarding the EWS default access.

 

Please see: https://www.veeam.com/kb4796

 

You’ll want to make sure EWS is explicitly enabled at an organisation and per mailbox level.

 

Later this year a patch to VB365 will swap over to Graph API from EWS. Trust me when I say you don’t want to be swapped over yet however as there are many limitations to the API that Microsoft are in the process of improving, such as the access to archive mailboxes, public folders etc.


  • Author
  • June 21, 2026

I am seeing this behavior only after user mailboxe are converted to shared mailbox. 

 


MicoolPaul
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Hi, noted and this doesn’t change my advice. You’re getting a forbidden message from Microsoft, likely because users that were licensed for EWS access no longer have that license. Please report back once you’ve followed the KB, waited an hour or so, and then retried the backup policy.