Your mail provider is refusing to send the email and says you are sending to to many people at once. I can not look up the limits your provider imposed because you simply do not mention their name. Perhaps look on their web site for email limits. The only other time I have seen this is when the SMTP server is incorrectly configured to not require a password, so as you are not logged in the limit is Zero. Check in your account settings >outgoing server (SMTP) and make sure a password is required to connect to the SMTP server and then that you know the password I guess.