The best way to report a suspicious or phishing email is to use the Phish Alert Button (PAB).  PAB is available in the installed version of Outlook, the web version of Outlook, and the mobile app version of Outlook.

Installed Version

In the installed version of Outlook, you can find the Phish Alert Button at the right of the ribbon menu.

Web Version

The web interface adapts to the resolution, window size, text size, which decides which icons are shown.  If there is sufficient space, you will see the Phish Alert icon which has a white envelope with an orange fishhook.  

If there is limited space you will not see the PAB.  To find it, click the 3 dots to the right of the sender (not the 3 dots on the top ribbon), then Apps, and you will find the Phish Alert button:

Mobile Version

In the Outlook mobile app on both iOS and Android, you can find the PAB by opening a message and tapping the three dots to the right of the sender. (Note: These are not the dots next to the icons, which bring up a different menu.) You may need to pull the menu up to see the options if they are off-screen.