How to "unjunk" an email in Office 365 (mark a message as not spam)
To indicate that a message isn't junk and allow the sender going forward:
- Click the Junk Email folder in the sidebar.

- Select the message or messages.

- Then select down carat next to the Report icon in the main toolbar, and select Not Junk from the drop-down that displays.

The message is moved to your Inbox. Subsequent messages from the same sender are no longer automatically marked as junk.