app2.umbler.com has no MX records.
Question and response
Question
dig app2.umbler.com. MX
Answer
The response has no answer records.
Authority
umbler.com. 60 SOA umblerdns03.westeros.local. admin.umbler.com. 2016065091 60 900 345600 60
Additional
. 0 OPT ; payload 4000, xrcode 0, version 0, flags 0