This message can't be sent. You've reached the limit on the number of messages that can be sent in a 24-hour period

I have an inuser that cannot send mail.  The error is:

 -This message can't be sent. You've reached the limit on the number of messages 
   that can be sent in a 24-hour period

I have looked in the sent iteam and there is only one item listed that was sent Tuesday @ 11ish am.  I do not understand why he is getting the error because it has been over 24hrs. 

Is there a way that I can refresh?

Thank you

Matthew