Unable to send message;Reason:554 5.1.8 Email Outgoing Blocked

Unable to send message;Reason:554 5.1.8 Email Outgoing Blocked

I got this error message while i am doing for a survey mail. Now, I knew that i can't do this , i try to unblock user but system siad operation permitted only paid user, why? I wait for more than 6hours but everything still unblock and can't unblock again. 

what should i do?