UNDELIVEREDMAIL365

The email is not legitimate, so you can safely conclude that this email should be ignored. 

First navigate to your EAC > Protection > Quarantine and verify if the emails are present. 

If you see some emails in Quarantine  you can check why they went to Quarantine by performing the following steps" 

1) I recommend getting the header of the email that you received and analyze it: https://testconnectivity.microsoft.com/   > header analysis. 

2) Check Anti-spam message headers: 

https://docs.microsoft.com/en-us/office365/securitycompliance/anti-spam-message-headers

This will tell you why emails are marked as spam and are present in the Hosted Quarantine. 
Hope this helps.

Clint Oliveira