stackdns-com.mail.protection.outlook.com has no MX records.
Question and response
Question
dig stackdns-com.mail.protection.outlook.com. MX
Answer
The response has no answer records.
Authority
mail.protection.outlook.com. 3600 SOA ns1-proddns.glbdns.o365filtering.com. hostmaster.o365filtering.com. 2013010801 3600 600 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0