mx.nifty.co.jp has no CNAME records.
Question and response
Question
dig mx.nifty.co.jp. NS
Answer
The response has no answer records.
Authority
nifty.co.jp. 2290 SOA odns0.nifty.ad.jp. hostmaster.nifty.ad.jp. 2026021703 28800 14400 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0