Mail Outgoing blocked - How to unblock?

Mail Outgoing blocked - How to unblock?

I just received the message below for one of my staff, how do I get her unblocked?  When I tried to do so in control panel, it says the feature is only available to paid accounts?

User : anneintong@holisticwebpresence.com

Details :Mail Outgoing blocked. Reason :Daily Mail Bounce Count Exceeds Limit [ Bounce count=20 ]