stamp.nttnavi.co.jp has no MX records.
Question and response
Question
dig stamp.nttnavi.co.jp. MX
Answer
The response has no answer records.
Authority
nttnavi.co.jp. 600 SOA ns6-tk01.ocn.ad.jp. postmaster.nttnavi.co.jp. 2025111801 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0