Server error: '550 Failed to Send - You have reached your Outbound Mail Limit for the hour'

I am receiving this error message for every e-mail I am trying to send:  Server error: '550 Failed to Send - You have reached your Outbound Mail Limit for the hour'

It has been happening all day, not jest during a 1-hour timeframe.  I had this issue in the past, but it is back.  I need to get this remedied.  Please advise.