sms.inext.net.mx has no MX records.
Question and response
Question
dig sms.inext.net.mx. MX
Answer
The response has no answer records.
Authority
inext.net.mx. 86400 SOA idcdns1.inext.net.mx. hostmaster.inext.net.mx. 2012080100 10800 3600 1209600 86400
Additional
. 0 OPT ; payload 2800, xrcode 0, version 0, flags 0