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Event ID: 1100 Source: MSExchangeActiveSync

Level
Description
Exchange ActiveSync device requests for your users are being blocked. This problem frequently occurs when the HTTP OPTIONS method request isn't allowed by the firewall. Please check the firewall that filters requests in front of your Client Access server and the Microsoft-Server-ActiveSync virtual directory.
Comments
 
Verify that the users affected are not members of the Administrators group. By design, the members of this group are not able to use the ActiveSync feature.
EV100212 (Microsoft Exchange Remote Connectivity Analyzer) can be used to test Exchange connectivity.
From a support forum:
I seem to have resolved it by removing the ActiveSyncVirtualDirectory and recreating it. I did this by running these commands in Exchange Management Shell:

Remove:
get-activesyncvirtualdirectory -server exch2010fe1 | Remove-ActiveSyncVirtualDirectory

Re-add:
New-ActiveSyncVirtualDirectory -WebSiteName "Default Web Site" -ExternalURL https://webmail.site.com/Microsoft-Server-ActiveSync -InternalURL https://exch2010fe1/Microsoft-Server-ActiveSync

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