bg3.exmail.qq.com has no CNAME records.
Question and response
Question
dig bg3.exmail.qq.com. NS
Answer
The response has no answer records.
Authority
exmail.qq.com. 30 SOA ns-tel1.qq.com. webmaster.qq.com. 1768811932 300 600 86400 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0