gslb4.comcast.com has no MX records.
Question and response
Question
dig gslb4.comcast.com. MX
Answer
The response has no answer records.
Authority
gslb4.comcast.com. 60 SOA gtd04-d.northlake.il.ndcchgo.comcast.net. hostmaster.gtd04-d.northlake.il.ndcchgo.comcast.net. 2026022201 10800 3600 604800 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0