mona1.singtel.com has no MX records.
Question and response
Question
dig mona1.singtel.com. MX
Answer
The response has no answer records.
Authority
singtel.com. 600 SOA dnspri.singnet.com.sg. hostmaster.singnet.com.sg. 2025100969 3600 1800 604800 28800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0