Check out this article on ClickZ by Stefan Pollard called "Marketers Still Unclear on Authentication" for a better understanding of this concept. Stefan defines authentication as the following:
Authentication helps you, as an e-mail sender, prove you are who you claim to be and that you have the right to send e-mail from your IP address. It's designed to block fraudulent e-mail that forges its identity or hijacks someone else's server or IP address to send e-mail.
You should ensure that your email provider has taken all of the necessary precautions to ensure that the email you send to your customers and prospects follows the authentication best practices.