mail3.labmuy.com has no CNAME records.
Question and response
Question
dig mail3.labmuy.com. NS
Answer
The response has no answer records.
Authority
com. 251 SOA a.gtld-servers.net. nstld.verisign-grs.com. 1771737381 1800 900 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0