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1500 characters remaining HTTP Error 451
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If the client is attempting to connect to the wrong server (that is, a server that cannot access the user's mailbox), or if there is a more efficient server to use to reach the user's mailbox, then a 451 Redirect error is returned.

The error returned by the wrong server resembles the following:

OPTIONS /Microsoft-Server-ActiveSync
Content-Type: application/vnd.ms-sync.wbxml
MS-ASProtocolVersion: 14.0

HTTP/1.1 451
Date: Tue, 08 Dec 2009 19:43:24 GMT
Server: Microsoft-IIS/7.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
X-MS-Location: https://mail.contoso.com/Microsoft-Server-ActiveSync
Cache-Control: private
Content-Length: 0

If an X-MS-Location header is present in the response, all subsequent requests SHOULD use the URL specified within the X-MS-Location header. If the server does not provide an X-MS-Location header in its response to the client, then the full Autodiscover command process is followed, as specified in [MS-ASCMD].

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