Houston1.Level3.net has no MX records.
Question and response
Question
dig Houston1.Level3.net. MX
Answer
The response has no answer records.
Authority
Houston1.Level3.net. 3600 SOA ns2.Level3.net. dns.level3.net. 2026030604 7200 600 2592000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0