How to fix slow Outlook IMAP folder synchronization issues

There could be a number of possible issues related to a mailbox not syncing properly when using Outlook 2013. Below are some common techniques you can try to resolve these kinds of issues.

First Option - Root your mailbox
----------------------------------------------
Unfortunately, Outlook doesn’t detect this automatically but you can set it manually.
To set the root folder path for your IMAP account:
1. File > Account Settings > Account Settings…
2. Double click on your IMAP account.
3. Click on "More Options…"
4. Click on the Tab "Advanced"
5. Option: Root folder path, this should just show "Inbox"
6. Click "OK" to exit windows.

Second Option - Sync groups
----------------------------------------------
Outlook 2013, perform the following steps:

1. Click the "Send/Receive" tab.
2. Click "Send/Receive Groups"
3. Click "Define Send/Receive Groups."  It should be the second option from the bottom.
4. Click "New..."
5. Type in a Name for the Group
6. Click OK.
7. Choose the IMAP account on the left.
8. Check the "Include the selected account in this group" box.
9. Make sure "Send mail items," "Get folder unread count for subscribed folders," and "Receive mail items" are checked and "Download complete items including attachments for subscribed folders" is selected.
10. Repeat steps 7-9 for each IMAP account if you have more than one.
11. Click "OK."
12. Select the new Group Name and make sure "Include this group in send/receive (F9)." and "Schedule an automatic/send receive every 30 minutes."(Default is 30, however you can reduce this to 1) are checked. Under "When Outlook is Offline" Check "Include this group in send/receive (F9.)"

Steps 13-16 are optional if the new group you created does not send/receive.
    
13. Select "All Accounts."
14. Click "Edit..."
15. Choose every IMAP account in the left and uncheck "Include the selected account in this group" box.
16. Click "OK."
17. Finally, click "Close"

Third Option - Subscribe folders
----------------------------------------------

1. From the Mail Section in outlook, Right click on "INBOX"
2. Select the option "IMAP folders..."
3. Click "Query"
4. A list of folders will generate below. You will notice 2 tabs above the list of folders, "All" and "Subscribed". Under the "All" Tab, it's important you see a folder Icon for each of those folders. If the Icon is missing you should not see that folder under the "Subscribed" Tab. The Goal is to make sure both Tabs match and have the folders you want to sync listed.
5. If any folders are missing the Icon you can select that folder and then click on "Subscribe"
6. Once complete, click "OK".

If you still have issues with syncing, please make feel free to contact support or create a ticket