Could be the Get folder unread option:-
- Go to “File -> Options”
- Select “Advanced”
- Click the “Send/Receive…” button under the “Send and receive” section
- Select the “All Accounts” group and click the “Edit…” button
- Uncheck “Get folder unread count for subscribed folders”
- Select “Use the custom behavior defined below” option are under “Receive mail items”
- In the “Select folders from the selected account to include in send/receive” box, check any folders that automatically receive mail. This would include inbox and any others folders that may have content filters that automatically deliver new messages to them.
- Close Outlook and re-open
- Mail should now send and receive.