Email won't load

Hughesnet Email  won't load at all now. I get  this message :

HTTP ERROR 504

Problem accessing ZCS upstream server. Reason: Cannot connect to the ZCS upstream server. Connection timeout.
Possible reasons:

• upstream server is blocked by a firewall
• upstream server is failing to send back the response in time
• upstream server is down

Please contact your ZCS administrator to fix the problem.

   My data allowance is low but not all used up