qq-b.mail.qq.com has no MX records.
Question and response
Question
dig qq-b.mail.qq.com. MX
Answer
The response has no answer records.
Authority
mail.qq.com. 180 SOA qq.com. webmaster.qq.com. 1768972738 300 600 86400 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0