st-web.cust.smartspb.net has no MX records.
Question and response
Question
dig st-web.cust.smartspb.net. MX
Answer
The response has no answer records.
Authority
cust.smartspb.net. 3600 SOA ns4.smartspb.net. hostmaster.smartspb.net. 52556 28801 1800 1209600 28800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0