Returned Emails?:

Email sent from your computer is verified by our email servers before it can be delivered. This verification process checks the validity of the email address, and the receiving ISP. If there is a problem with any of the components of the email address, it will be returned and generate a mailer-daemon. Please take note of what the contents of the mailer demon as it will list the reason the email was undeliverable.

The majority of the time reading the mailer-daemon will help you access the problem and resend your email.