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