There might be a number of possible issues related to mailbox not syncing properly when using Outlook. This could be due to the large number of mailbox you have in your IMAP accounts.
- Launch your Outlook
- Click the Send/Receive tab.
- Click Send/Receive Groups
- Click Define Send/Receive Groups. It should be the second option from the bottom.
- Click All Accounts
- Click Edit
- Choose the IMAP account that you are experiencing problem with on the left.
- Click on Use the custom behavior defined below and tick only Inbox, Drafts, Sent, Trash, Junk Email and leave the other sub-folder unchecked
- Click "OK."
- Ensure that Schedule an automatic send/receive every X minutes is ticked and set to 5 minutes
