please check your exchange server configuration:
I agree with Daniel and Enrique.
I would to add that this is a normal SMTP servers configuration added to avoid that the server becomes a spam relay. So be aware about what you are doing if you remove/change this limitation.
There are third party SMTP server configured for mass mailing. You could evaluate to buy services from them instead of change your server configuration. You can search a supplier by using Google. These mass mailing SMTP server are configured in order to avoid spam but be able, at the same time, to send a lot of email messages in a small time.