mxbackup03.mail.esat.net has no CNAME records.
Question and response
Question
dig mxbackup03.mail.esat.net. NS
Answer
The response has no answer records.
Authority
esat.net. 10800 SOA auth02.ns.esat.net. hostmaster.esat.net. 2026021801 28800 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0