ns5.smartspb.net has no MX records.
Question and response
Question
dig ns5.smartspb.net. MX
Answer
The response has no answer records.
Authority
smartspb.net. 600 SOA ns4.smartspb.net. hostmaster.smartspb.net. 928 28800 1800 1209600 28800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0