mta3.email.microsoftonline.com has no MX records.
Question and response
Question
dig mta3.email.microsoftonline.com. MX
Answer
The response has no answer records.
Authority
email.microsoftonline.com. 3600 SOA ns1.exacttarget.com. hostmaster.exacttarget.com. 2025010702 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0