1. The Content-type must be set to text/plain , NOT text/html
2. Emails sent to Hotmail are bounced back since they have blacklisted x10hosting's mail server. Other email providers may do the same.
3. mail() does not seem to work run as a cron job.
Alternatives are to set up a Google Apps account for your domain and use PHPMailer to send the emails thru it via SMTP.