How-to fix slow Outlook IMAP Subscribed Folders Synchronization outlook 365

1. Select your Inbox
   
2. From the menu click Folder > IMAP Folders
   
3. Click Query
4. Select all your IMAP folders
5. Click Unsubscribe. The outlook may warn you some folders are ‘special’. Click OK if this warning appears
6. Click OK
7. Close and re-open Outlook.
8. Select your Inbox
9. From the menu click Folder > IMAP Folders
10. Click Query
11. Select all your IMAP folders
12. Click Subscribe
13. Uncheck When displaying hierarchy in Outlook, show only subscribed folders.
14. Click OK

Outlook will now synchronize your folders, the time this takes depends on the size and number of emails stored in each of your IMAP folders.