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Adjusting the maximum number of open objects that a MAPI client can use at the same time


Click on one of the below problems to get a solution:


MAPI session limits.

In Exchange server the number of message objects that are allowed to be opened by a single MAPI session is limited to 250 objectsThis limitation can cause some problems with syncing folders that contain large amount of the elements. For example this problem may occur after the user has changed the category of 500 elements  (in one step) in the source folder. Because of the limitation described above the part of changed elements will not be synchronized into the target folder and in the log files the following error occurs:

Error: Failed to open message from table notification. MAPI_E_CALL_FAILED


Error: Failed to open message from table notification. MAPI_E_TOO_BIG

This problem can be solved by adding the value in the Registry Editor with the new number of maximum messages objects that are allowed to be opened in a single MAPI session. Make sure that the account you use belongs to the local Administrators group and then follow these steps:

  1. Go to the Registry Editor (regedit) of each of your MS Exchange Servers where MS Exchange Information Store service is running (Mailbox role servers).
  2. Locate the registry key:
  3. Right click ParametersSystem, choose New, then click Key.
  4. Type MaxObjsPerMapiSession and press ENTER.
  5. Now right click MaxObjsPerMapiSession, choose New and click DWORD Value.
  6. Type the object type (for example objtMessage) and press ENTER to name the entry that changes the default maximum of the objects.
  7. Right click the entry that you created and click Modify.
  8. Go to the Value data box and type the new maximum number of objects to which you want to limit this entry, then click OK.
  9. Apply changes:
    1. Exchange Server 2003: Reboot the machine.
    2. Exchange Server 2007: Changes should be recognized automatically after 5 minutes, if not reboot the machine.
    3. Exchange Server 2010 and newer: Restart MS Exchange Information Store service or reboot the machine.

 You can learn more from the articles on Microsoft Technet:

If you try to synchronize more than 250 mailboxes you may notice some of them are not synchronized and in the log files you find entries that contain the below phrase:


To fix this issue, follow the steps below:

  1. Go to the Registry Editor (regedit) of your machine where the Exchange Sync is installed.
  2. Locate the registry key:
    • In 32bit operating systems:
    • In 64bit operating systems:
  3. Right click on it, choose New, then click DWORD Value.
  4. Name it UseSeparateRPCConnections and press ENTER.
  5. Now right click on it, choose Modify:
    1. change Base to Decimal
    2. and modify Value Data to 1.
  6. Click OK to confirm changes.
  7. Restart the synchronization service.
  8. Go through the service log files and check if you see the following entry:
    Profile section set with CONNECT_USE_SEPARATE_CONNECTION flag
  9. Reboot the machine.

See also: disabling MAPI session limit and NSPI connection to a Windows-based domain controller causes MAPI client applications to fail and returns a "MAPI_E_LOGON_FAILED" error message.

MS Exchange Server 2010 and 2013/2016 Throttling settings.

Regardless of the above described MAPI limits, MS Exchange Server 2013 will apply throttling to CodeTwo Exchange Sync synchronization actions. Similar issue may also occur in some installations of Exchange Server 2010. Such limits are becoming visible if 32 or more folders are configured in your synchronization tasks. This issue is visible in the software service log files as following error messages logged for mailboxes above the limit:

Error: Failed to open message store "<MailboxName>\<FolderName>"/ MAPI_E_FAILONEPROVIDER (0x8004011d). module: FoldersGroupCreator.cpp, line: 259, func: CFoldersGroupCreator::operator ()

To overcome this issue an administrator has to increase RcaMaxConcurrency limit. This problem is described in this and this MS Technet articles. Below we present an example of how to apply unlimited RcaMaxConcurrency settings.

  1. Log on to your MS Exchange Server machine.
  2. Run ems Exchange Management Shell.
  3. Execute the below cmdlet to create a new throttling policy. Change "YourNewPolicyName" to any name of your choice.
    1. Exchange Server 2010:
      New-ThrottlingPolicy -Name "YourNewPolicyName" -RCAMAXConcurrency $null
    2. Exchange Server 2013/2016:
      New-ThrottlingPolicy -Name "YourNewPolicyName" -RCAMAXConcurrency Unlimited
  1. Execute the below cmdlet to assign newly created throttling policy to a mailbox of a user whose account will be used by the synchronization service. Change "MailboxName" to the name of the desired mailbox.
    Set-Mailbox -identity "MailboxName" -throttlingpolicy "YourNewPolicyName"
  2. Restart CodeTwo Exchange Sync synchronization service.
  3. For more details please refer to above linked MS Technet articles.
  • Applies to: CodeTwo Exchange Sync
  • Categories: Troubleshooting
  • Last modified: 2016-06-09
  • Created: 2010-03-18
  • ID: 137
  • Keywords: obejcts, limitation, limits, limited, MAPI, error, errors
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