1243.static.theplanet.com has no MX records.
Question and response
Question
dig 1243.static.theplanet.com. MX
Answer
The response has no answer records.
Authority
1243.static.theplanet.com. 3600 SOA ns1.theplanet.com. root.1243.static.theplanet.com. 2018022300 3600 300 604800 3600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0