39119: Acronis Backup Advanced 11.7/11.5: Mailbox Backup Fails with "Cannot Open Public Folder or Mailbox"

use Google Translate

Last update: Wed, 2018-09-12 15:08

Mailbox backup fails with error "Cannot open public folder or mailbox '<mailbox>'".


The mailbox-level backup of several folder items sometimes fails with following error message:

Cannot open public folder or mailbox '<mailbox>'.


The Acronis Agent for MS Exchange is using the MAPI protocol (Messaging Application Programming Interface) for communication. It is the same protocol, that MS Outlook is using to communicate with MS Exchange server.

MAPI has a limit of concurrent requests to MS Exchange database which is inherited from MS Exchange throttling policy. Throttling policy is used to help prevent MS Exchange clients sending too many requests to the server causing decrease of server performance.

When selecting many different folders to backup, the Acronis Agent for MS Exchange may fail to resolve all the items because throttling policy does not let all necessary MAPI requests to be sent.

You can find more detailed information about the throttling polices in the following Microsoft Technet article: Understanding Client Throttling Policies.


You can work around this limitation by using one of the two offered solutions:

  1. Run backup of the whole mailbox(es) instead of selecting many individual sub-folders, or
  2. Adjust MS Exchange throttling policy to disable throttling of the user account that you specify in backup plan to connect to the MS Exchange server:
    1. Start MS Exchange Management Console;
    2. Issue the following command to create a new throttling policy:

      New-ThrottlingPolicy AcronisPolicy -RCAMaxConcurrency $null -RCAPercentTimeInAD $null -RCAPercentTimeInCAS $null -RCAPercentTimeInMailboxRPC $null

    3. Associate new policy with the mailbox of the user account that you specify in backup plan to connect to the MS Exchange server:

      Set-Mailbox <MAILBOX> -ThrottlingPolicy AcronisPolicy

      (!) MAILBOX is the mailbox associated with the user account that is used to access Exchange server in ABR backup plan.

    4. Wait some time till new policy gets enabled (sometimes it can take a while - if you do not want to wait, you can force update of the policy by restarting the Microsoft Exchange RPC Client Access service on all Exchange CAS servers). You can use the following command to check if new policy was already applied to the user account or not:

      Get-ThrottlingPolicyAssociation <USERNAME>

      Policy associated with the specified account will be shown in ThrottlingPolicyID field - it should indicate 'AcronisPolicy' as associated policy.

    5. Run the backup plan again.

If the issue is not solved, please create a system report of the affected machine and contact support