mail.tec.com has no CNAME records.
Question and response
Question
dig mail.tec.com. NS
Answer
The response has no answer records.
Authority
tec.com. 1503 SOA hans.ns.cloudflare.com. dns.cloudflare.com. 2396893512 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0