this usually happens with exchange email servers when they overloaded. like Chris has suggested, increasing the time between sending emails help to prevent that, however, if other users on your host are sending emails using the same SMTP server, that may cause the SMTP Server to be overloaded as well.
At the end of the day, it's an issue with the SMTP server. if increasing the wait time does not work, you would need to use a different SMTP server.