The following recipient(s) cannot be reached: Server error: '451 4.7.1

Hi Everyone,

I am getting this error:

Your message did not reach some or all of the intended recipients.

     Subject:    Work

     Sent:27/02/2013 16:19

The following recipient(s) cannot be reached:

      on 27/02/2013 16:19

           Server error: '451 4.7.1 This message would exceed your limit of 1000 recipients per 24 hours. In the last 24 hours you have sent messages to 1001 recipients. (1)'

I have not sent an Email in the past 24 hours and it is telling me I have sent too many?

Any help?