NDR 554 5.4.4 SMTPSEND.DNS.MxLoopback
A little bit of history on why I was receiving this DNS loop error NDR email (NDR 554 5.4.4 SMTPSEND.DNS.MxLoopback). We have a Hybrid solution with Exchange 2013, DirSync and Office 365. I created a new Office365 user from my on premise Exchange server and Dirsynced the account to the cloud and gave it a license. We then have an internal SharePoint workflow than emails the account when a specific criteria was met, this is what would trigger the loopback NDR email.
The fix was as follows for me. I logged into my local Exchange Admin Console and edited the account in question and went to the email address field. When the account got created it had populated the “Set as remote routing address” field to the users default SMTP address. Changing this to the mail.onmicrosoft.com account then let the internal email route/relay to outside without giving me the loopback NDR bounce back.
I also have under my accepted domains mail.onmicrosoft.com set and an Internal relay as the domain type.