Click Admin, and then click Exchange to open the Exchange admin center.
In the left navigation pane, click recipients, and then click mailboxes.
In the list of mailboxes, double-click the user, and then click mailbox features.
Under Mobile Devices, take the following actions:
If you see Disable Exchange ActiveSync, this means that ActiveSync is enabled for the user. Go to "Step 2: Confirm that the mobile device isn't blocked by an ActiveSync quarantine rule."
If you see Enable Exchange ActiveSync, this means that ActiveSync isn't enabled for the user. Click Enable Exchange ActiveSync, click Yes when you're prompted, and then click Save.
After you re-enable ActiveSync, try to set up the device again.
Step 2: Confirm that the mobile device isn't blocked by an ActiveSync quarantine rule
In the Exchange admin center, click mobile, and then click mobile device access.
Confirm that the user's mobile device isn't in the list of quarantined devices.
Step 3: Confirm that ActiveSync can be set up by using Autodiscover
The Autodiscover service makes it easier to set up Outlook and mobile phones. The Autodiscover service uses a user's email address and password to automatically set up a user's profile.
To troubleshoot this issue further, run the Exchange ActiveSync Autodiscover test in Microsoft Remote Connectivity Analyzer. If the user is using a local wireless network to connect to Exchange Online, the user should run both tests to make sure that the local network allows for connections to the ActiveSync endpoints.
Test Exchange Online ActiveSync access externally by using Remote Connectivity Analyzer
Note Admins may want to guide users through running Remove Connectivity Analyzer if this is necessary. The Exchange ActiveSync Autodiscover test requires the user to enter his or her credentials.
In a web browser, browse to the Microsoft Remote Connectivity Analyzer tool at the following website:
Step 4: Set up the mobile device without using Autodiscover (if you don't want to use Autodiscover)
We recommend that you use Autodiscover when you try to set up mobile devices. However, if you want to set up a mobile device without using Autodiscover, use the procedure that is described on one of the following Microsoft websites:
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