smtp03.topdns.com has no MX records.
Question and response
Question
dig smtp03.topdns.com. MX
Answer
The response has no answer records.
Authority
topdns.com. 3600 SOA ns-uk.topdns.com. hostmaster.topdns.com. 2026021803 43200 900 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0