We set up our servers to prevent anyone to use our SMTP server. To send email, you need to set up SMTP Authentication in your mail client. The password is as same as your POP3 accounts', more details can be viewed here. Another reason could be that you have used up your daily sending limit, you can check this by going to this page in DirectAdmin.
Furthermore, we have a system in place that whenever failed login attempts are logged, the IP will be blocked for 24 hours.