The server has reached its currently configured outstanding requests limit of [] request(s). At this point, all new requests will be returned immediately with HTTP 503 (Service unavailable) error messages until the pending requests are processed. For more information about how to increase this limit, see Microsoft Knowledge Base article 901382 “Mobile Device Users Cannot Synchronize their Devices and Receive HTTP 503 (Service unavailable) Messages” (http://go.microsoft.com/fwlink/?linkid=3052kbid=901382).

The server reached the currently configured limit for concurrent Exchange ActiveSync connections. All new requests will be returned immediately with HTTP 503 (Service unavailable) error messages until the pending requests are processed. The limit that is currently configured is specified in the text of the event. The default limit for concurrent Exchange ActiveSync connections is 10,000. You must take immediate administrator action to ensure ActiveSync availability.

• Deploy an additional front-end server to minimize the load. This option is only applicable in front-end and back-end server deployments that use network load balancing solutions such as Windows Network Load Balancing.
• Increase the concurrent request limit.

Important  This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore the registry if a problem occurs. For information about how to restore the registry, view the “Restore the Registry” Help topic in Regedit.exe or Regedt32.exe.

To increase the concurrent request limit:

1. On the computer running Exchange Server, start Regedit.exe.
2. Open the following registry key:

   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MasSync\Parameters

   The registry value name for the concurrent request limit is MaxClients.

3. Do one of the following:

   • If the registry parameter already exists, to increase the maximum heartbeat interval value, right-click MaxClients, and then click Modify. In Edit DWORD Value, in Value data, type a value larger than the current setting.
   • If the registry key parameter does not exist, to add the registry key parameter and specify a larger value than the default of 10000 connections, right-click Parameters, point to New, and then click DWORD Value. For the new value name, type MaxClients. Double-click MaxClients. In Value data, type a value larger than 10000. Click OK.
4. Close Registry Editor.
5. Restart Internet Information Services (IIS).

Before you edit the registry, and for information about how to edit the registry, see Microsoft Knowledge Base article 256986, “Description of the Microsoft Windows Registry” (http://go.microsoft.com/fwlink/?linkid=3052&kbid=256986).

For more information about how to resolve this error, see Microsoft Knowledge Base article 901382, “Mobile device users cannot synchronize their devices and receive HTTP 503 (Service unavailable) messages” (http://go.microsoft.com/fwlink/?linkid=3052&kbid=901382).