防止通过PHPMailer发送的订单确认结束垃圾邮件

When I send order confirmations via PHPMailer, they usually end up in spam folders of Google, Yahoo and Hotmail and also take a long time to deliver. Is there a checklist or something of things I should be aware of when sending confirmations this way? Mail from most sites that I order from or register to gets delivered instantly and doesn't get flagged as spam. What do they know that I don't?When I send order confirmations via PHPMailer,