Exceeding daily limit

I've started getting this message :- Sending of the message failed. An error occurred while sending mail. The mail server responded: User let@slugletting.com has exceeded its 24-hour sending limit. Messages to 500 recipients out of 500 allowed have been sent. Relay quota will reset in 4.94 hours.. Please verify that your email address is correct in your account settings and try again. I've never sent that many emails and it's happening every day