When you try to sync a mobile device that's using Microsoft Exchange ActiveSync (EAS) with Microsoft Exchange Online in Microsoft Office 365, your device can't connect. Depending on your device, you may get one of the following error messages:
Cannot Get Mail The connection to the server failed.
Cannot Get Mail An error occurred while delivering the message.
Unable to open server connection due to a security update.
If you use the Exchange admin center to view the mobile device details or if you collect a synchronization log for the device, you get the following error message:
AccessState : Access Denied
The Access set by column will contain additional info such as the following:
Too many identical commands
For steps that Office 365 admins can use to identify this issue in the Exchange admin center, see the "More Information" section.
When a mobile device acts in a manner that can adversely affect Exchange Online service performance, the device is put into an Access Denied state for a short time. For example, this occurs if a device sends too many identical sync commands to the service for a particular folder in a very short period.
Exchange Online has implemented EAS throttling to manage and maintain optimal performance of the Exchange Online environment in Office 365. For more info about how you can view the access state of the mobile device when you use the Exchange admin center and how to collect EAS logs, see the "More Information" section.
To work around this issue, follow these steps while you discuss this workaround with your device vendor or carrier:
When a delay in sync occurs, don't repeatedly press the Synchronize button on your mobile devices. Additionally, don't quickly switch folders and repeatedly press the button to sync.
Wait at least 10 minutes before you take additional action so that the throttling will be automatically removed by the service.
Switch to manual sync to see whether that works to connect and sync your device with the mailbox. Doing this helps prevent your device from sending too many identical commands.
If the problem persists, check wireless connectivity for the device in the local network. You may have to contact your device manufacturer or carrier to capture and review device logging.
If the issue isn't resolved, contact your mobile device manufacturer to report and investigate this issue.
Note The implementation of the EAS protocol on the device controls the frequency of commands sent to the service. Microsoft Support can't change the software on the devices. Any updates to the device can only occur through the device manufacturer.
For more information about how to collect EAS logs, see the following Microsoft Knowledge Base article:
2461792 How collect ActiveSync device logs to troubleshoot sync issues between mobile devices and Exchange Online
You can open the log file by using a text editor such as Notepad. In the log file, search for a 503 error that contains the following information:
ResponseHeader together with AccessState : Blocked
AccessStateReason : SyncCommands or AccessStateReason : RecentCommands
Be aware that, before this error, the device may send the same command many times. For example, the device may send the Sync command together with identical parameters, such as SyncKey and CollectionId.
If you examine the timestamps, you can see the commands that are sent repeatedly in a very short time period, which result in the 503 Service Unavailable error. This indicates that a device is performing in a manner that causes the Exchange Online service to enable the throttling protection to prevent the device from affecting the performance of the service.