slb.comcast.com has no MX records.
Question and response
Question
dig slb.comcast.com. MX
Answer
The response has no answer records.
Authority
slb.comcast.com. 900 SOA dns101.comcast.net. hostmaster.comcast.com. 55023 7200 3600 604800 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0