Explanation/Reference:
D: Office 365 Small Business admins can troubleshoot email delivery problems by using the Exchange Online message trace tool. The tool helps admins track specific messages sent in the past 90 days.
A: Office 365 users and admins can also use delivery reports in Outlook Web App to check the delivery status of messages they've sent or received. If you sent a message to five people, for example, you can check the status of the delivery of that message to each person.
Go to Delivery reports settings:
1. Sign in to Outlook Web App.
2. At the top of the page, select Outlook (or Mail). Or, select the App launcher , and then select Outlook.


3. At the top of the page, select Settings > Options.
4. In Options, select Organize email > Delivery reports. Or, in Options, under MAIL > Layout, select Delivery reports.
Reference: Troubleshoot email delivery using the Exchange Online message trace tool
https://support.office.com/en-au/article/Troubleshoot-email-delivery-using-the-Exchange-Online-message- trace-tool-e7758b99-1896-41db-bf39-51e2dba21de6
Reference: https://support.office.com/en-sg/article/Delivery-reports-6556a1d7-e988-4428-88de-
48a4ae58555f