i have get the following error using sandbox

HTTP/1.1 400 Bad Request Date: Fri, 17 Mar 2017 07:05:58 GMT Server: Apache Content-Length: 28 Connection: close Content-Type: text/html; charset=iso-8859-1 Http/1.1 Service Unavailable

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If your building HTTP Headers make sure you terminate each one with a CRLF (\r\n), recently the server got more strict and requires you to follow http spec exactly.