mx.plustariff.com has no CNAME records.
Question and response
Question
dig mx.plustariff.com. NS
Answer
The response has no answer records.
Authority
plustariff.com. 3478 SOA dns.plustariff.com. hostmaster.plustariff.com. 1206848012 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0