The php mail function works great for a contact page on your site that will send the email to your sites MX. However, when sending to other mail servers such as yahoo, gmail, hotmail... it's not reliable enough to trust. Why? Because the mail function doesn't have SMTP authentication. Use one of php's prebuilt libraries, such as class.phpmailer.php. I use it for a mailing list and have tested through Yahoo, Gmail, Hotmail, and several others. It also provides a way to send both a plain text and html version of the message.
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