_netblock4.teco.com.ar has no MX records.
Question and response
Question
dig _netblock4.teco.com.ar. MX
Answer
The response has no answer records.
Authority
teco.com.ar. 300 SOA jazzy.arnet.com.ar. dnsmaster.arnet.net.ar. 3130001578 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0