3.1.5.2.1 HTTP Error 401, 403, and 500
If the server responds to any command with an HTTP error 401, 403, or 500, the client SHOULD send an Autodiscover command request ([MS-ASCMD] section 2.2.1.1) to the server.
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If the server responds to any command with an HTTP error 401, 403, or 500, the client SHOULD send an Autodiscover command request ([MS-ASCMD] section 2.2.1.1) to the server.