mail5.questback.net has no MX records.
Question and response
Question
dig mail5.questback.net. MX
Answer
The response has no answer records.
Authority
questback.net. 3600 SOA ns1.dnshost.net. zonemaster.dnshost.net. 1705568482 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0