ns6.166.com has no MX records.
Question and response
Question
dig ns6.166.com. MX
Answer
The response has no answer records.
Authority
166.com. 60 SOA ns4.166.com. admin.166.com. 991188 36000 3000 3600000 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0