Problem: Emails sent via ExpressionEngine are never reaching their destination.

Solution: There are a few troubleshooting steps to take in this instance.

Go to Admin -> System Preferences -> Email Configuration and check that Return email address for auto-generated emails contains a valid e-mail address.

Go to Admin -> System Preferences -> Email Configuration and change Enable Email Debugging? to “On”
Click the Communicate tab and attempt to send an e-mail.  If the sending fails it will output an error that will assist you in troubleshooting the problem.

You can also try changing the method of sending email in Admin -> System Preferences -> Email Configuration Email Protocol

If Enable Debugging is on and you are not receiving any error messages it means that EE is successfully opening a socket to your email server, writing the email headers and data to the mail queue, and closing the connection.  In this case, the next step is to contact your hosting provider or server administrator for further troubleshooting.

Further Information: Forum Thread