ns-e.b.isi.edu has no MX records.
Question and response
Question
dig ns-e.b.isi.edu. MX
Answer
The response has no answer records.
Authority
b.isi.edu. 7200 SOA ns-w.b.isi.edu. b-poc.isi.edu. 1752845783 7200 7200 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0