It is a known issue of Outlook 2013/2016 connecting via IMAP.
- mail isn't populating
- Sent Items, Outbox, Junk Email folder etc. show up as the subfolders of the Inbox folder.
Until Microsoft releases an update to fix the issue, use the following workaround:
- In Outlook 2013/2016, navigate to Home > Info > Account Settings > Account Settings:
- Select the IMAP account and click Change (or double-click the IMAP account name)
- Navigate to More Settings > Advanced and enter INBOX in the Root folder path field
- Click OK > Next > Finish
All default folder will show up as top-level ones and mail will start populating to the Inbox folder.