ns4.purdue.edu has no MX records.
Question and response
Question
dig ns4.purdue.edu. MX
Answer
The response has no answer records.
Authority
purdue.edu. 3600 SOA ns3.purdue.edu. hostmaster.purdue.edu. 716087205 1200 180 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0