imp-server4.us.impsat.net has no MX records.
Question and response
Question
dig imp-server4.us.impsat.net. MX
Answer
The response has no answer records.
Authority
impsat.net. 3600 SOA ns1.us.impsat.net. hostmaster.impsat.net. 1764832216 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0