The MS-ASProtocolVersion header indicates the protocol version that the client is using to format the request. This header SHOULD NOT be used if the base64-encoded query value is being used. Instead, the Protocol version field of the base64-encoded query value SHOULD be set, as specified in section

The following values, which correspond to the ActiveSync protocol versions, are valid: "16.1", "16.0", "14.1", "14.0", "12.1", "12.0", and "2.5". The latest version is 16.1.