寄信到 hotmail.com 收到退信如下：
Your message did not reach some or all of the intended recipients.
Subject: Test mail
Sent: 2012/9/13 上午 10:08
The following recipient(s) cannot be reached:
firstname.lastname@example.org on 2012/9/13 上午 10:05
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
< mail.mydomain.com #5.0.0 SMTP; 550 SC-001 (SNT0-MC1-F7) Unfortunately, messages from xx.xx.xx.xx weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.>
按照 http://mail.live.com/mail/troubleshooting.aspx#errors 裡面提到的 DNS 設定，新增一筆 mydomain.com IN TXT “v=spf1 ip4: xx.xx.xx.xx/32 -all”。經過幾個小時讓 DNS 更新到 Internet 後，再次寄信到 hotmail.com 就不會退信了。