mail.o2.com has no CNAME records.
Question and response
Question
dig mail.o2.com. NS
Answer
The response has no answer records.
Authority
o2.com. 669 SOA n1.02.net. hostmaster.02.net. 2026022401 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0