mta4.nelnet.net has no MX records.
Question and response
Question
dig mta4.nelnet.net. MX
Answer
The response has no answer records.
Authority
nelnet.net. 3600 SOA pdns03.domaincontrol.com. dns.jomax.net. 2026022000 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0