Certain hosting providers do not play nicely with the WordPress mail functions. There have been several plugins developed to work around certain hosting providers. At the time of writing, we think this is the best solution:
WP Mail SMTP – fixes your email deliverability by reconfiguring WordPress to route all emails through a third party SMTP provider such as Gmail, Yahoo or any other popular email service. This will often result in more emails being properly delivered and less emails marked as spam by internet service providers.