ns4.he.net has no MX records.
Question and response
Question
dig ns4.he.net. MX
Answer
The response has no answer records.
Authority
he.net. 86400 SOA ns1.he.net. hostmaster.he.net. 202602210 10800 1800 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0