ns4.servernap.com has no MX records.
Question and response
Question
dig ns4.servernap.com. MX
Answer
The response has no answer records.
Authority
servernap.com. 3600 SOA ns1.servernap.com. hostmaster.servernap.com. 2021120800 3600 900 604800 86400
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0