Outgoing mail blocked

Outgoing mail blocked

I keep unblocking it but it keeps blocking again. This is the message I'm getting:  

Your Mail Outgoing blocked. Reason :Daily Mail Size Quota Exceeds Limit [Total Mail Size=307,200]


I have not sent any large email attachments since Tuesday (and that was only 6MB) and yet it is still blocked! Help - this is affecting my ability to run my business.


Mandy