Attempts to provision a submissions ID for the Symantec Messaging Gateway (SMG) appliance via the Control Center Spam->Submissions Settings page results in a failure to connect through the configured proxy server. The connection attempt is rejected by the configured proxy server with a 400 error.
curl: (56) Received HTTP code 400 from proxy after CONNECT
This is a known issues with some proxy servers. The submissions provisioning attempt contains a Content-Length HTTP header with a non-zero value. Some proxy servers will not accept this and reject the http CONNECT as malformed.
This issue has been addressed in SMG 10.5.2. Please update when able.
As a workaround, please consult your proxy server documentation and adjust the proxy configuration such that it will accept an http CONNECT method with a Content-Length greater than zero.
Symantec Messaging Gateway