sandbox.securemessage.us has no MX records.
Question and response
Question
dig sandbox.securemessage.us. MX
Answer
The response has no answer records.
Authority
securemessage.us. 14400 SOA dns1.cscdns.net. hostmaster.cscdns.net. 2025020311 28800 7200 604800 14400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0