smtp07.sieuthimaychu.us has no MX records.
Question and response
Question
dig smtp07.sieuthimaychu.us. MX
Answer
The response has no answer records.
Authority
us. 900 SOA a.cctld.us. admin.tldns.godaddy. 1772001869 1800 300 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0