mail3.knockbot.info has no MX records.
Question and response
Question
dig mail3.knockbot.info. MX
Answer
The response has no answer records.
Authority
knockbot.info. 300 SOA 01.dnsv.jp. hostmaster.dnsv.jp. 1752887637 3600 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0