mx1800R.dx.2iij.net has no CNAME records.
Question and response
Question
dig mx1800R.dx.2iij.net. NS
Answer
The response has no answer records.
Authority
dx.2iij.net. 4472 SOA dns0.iij.ad.jp. postmaster.iij.ad.jp. 100165 3600 1800 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0