ns2.amzndns.net has no MX records.
Question and response
Question
dig ns2.amzndns.net. MX
Answer
The response has no answer records.
Authority
amzndns.net. 907 SOA g-ns1.amzndns.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 907
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0