o3.clicksend.rest.com.au has no MX records.
Question and response
Question
dig o3.clicksend.rest.com.au. MX
Answer
The response has no answer records.
Authority
rest.com.au. 900 SOA ns-1532.awsdns-63.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0