outbound.protection.outlook.com has no MX records.
Question and response
Question
dig outbound.protection.outlook.com. MX
Answer
The response has no answer records.
Authority
outbound.protection.outlook.com. 60 SOA ns1-36.azure-dns.com. hostmaster.o365filtering.com. 2021562360 300 900 1209600 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0