Wrongly blocked for 'exceeding size quota'

Wrongly blocked for 'exceeding size quota'

My client n****@nickycoates.com is constantly getting this message:

Details :Mail Outgoing blocked. Reason :Daily Mail Size Quota Exceeds Limit [Total Mail Size=768,000] 

I've unblocked repeatedly but it keeps happening. She's not sending anywhere near that limit. Please advise.

thanks