mx5.sophos.com has no CNAME records.
Question and response
Question
dig mx5.sophos.com. NS
Answer
The response has no answer records.
Authority
sophos.com. 39 SOA asia1.akam.net. hostmaster.sophos.com. 2020012650 7200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0