u5.amazonaws.org has no MX records.
Question and response
Question
dig u5.amazonaws.org. MX
Answer
The response has no answer records.
Authority
amazonaws.org. 900 SOA dns-external-master.amazon.com. root.amazon.com. 2008040338 600 900 604800 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0