How to deploy and-configure the "Report Message" add-in

Deploy the add-in for users

Login to the Microsoft 365 admin center at https://admin.microsoft.com.
On the left nav, press Show All then expand Settings and select Integrated Apps.
On the page that loads, press Get Apps.
In the page that appears, in the top right Search box, enter Report Message or Report Phishing, and then select Search.
Press Get it now on your chosen app within the search results (publisher is Microsoft Corporation).
On the flyout that appears, select who to deploy the add-in to. If testing you may wish to use a specific group, otherwise configure it for the entire organization – when you’ve made a selection press Next.
Review the permissions, information and capabilities then press Next.
Press Finish deployment (it can take 12-24 hours for the add-in to appear automatically in Outlook clients).

Login to the Microsoft Security portal at https://security.microsoft.com.
On the left nav, select Settings and choose Email & collaboration.
Select User reported settings.
Ensure Microsoft Outlook Report Message button is toggled to On.
Under Send the reported messages to choose Microsoft (Recommended).
Ensure Let users choose if they want to report is unchecked and Always report the message is selected.
Press Save.

On the configuration page from the earlier steps, underneath the User reporting experience, configure the before and after reporting pop-ups title and body if desired. The end users will see the before reporting pop up if Ask me before reporting is also enabled.
If you wish for notifications to come from an internal organizational mailbox, select Specify Office 365 email address to use as sender and search for a valid mailbox in your organization to send the notifications from.
Press Customize notifications to set up the text sent to reporting users after admin reviews a reported message using Mark & Notify, configure the Phishing, Junk & No threats found options.
On the Footer tab, select the global footer to be sent for notifications, along with your organization’s logo if appropriate.