Outgoing mails are blocked

Outgoing mails are blocked

Outgoing mails are blocked for automated@naturenurture.lk and returning following error on each attempt

{ [Error: Message failed: 554 5.1.8 Sender Address Blocked]
  code: 'EMESSAGE',
  response: '554 5.1.8 Sender Address Blocked',
  responseCode: 554,
  command: 'DATA' }

How to fix this issue permanently ? 

This email id should handle around 100 emails a day.