smtp03.nbsvoice.com has no MX records.
Question and response
Question
dig smtp03.nbsvoice.com. MX
Answer
The response has no answer records.
Authority
nbsvoice.com. 100 SOA ns1.simlab.net. admins.hoon.simlab.net. 1708619326 28800 7200 1209600 100
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0