Otaku IT Guy!
Having many folders with many messages in Microsoft Outlook, especially if they have attachments, can be slow. This issue is because Microsoft Outlook downloads all messages every time including attachments versus a normal email IMAP application where only headers are synced to present crucial information only, and messages and attachments are ONLY downloaded and cached upon viewing the message or if the mail application is explicitly told to download all messages. Remedy this situation as follows:
After setting up your IMAP email account in outlook, right click on your email account and select “IMAP Folders…”
Click on the “Subscribed” tab and click the “Query” button.
Select all items in the list as follow:
Select the first item in the list and scroll all the way down.
Hold shift and click the last item in the list.
Click the “Unsubscribe” button.
It may prompt about folders that cannot be unsubscribed, just click “OK” here.
Select the “All” tab as we are now going to sync folders we wish or need to sync.
Select “Drafts” and click subscribe.
Select “Inbox” and click subscribe.
Select “Junk” and click subscribe.
Select “Sent” and click subscribe.
Select “Templates” and click subscribe.
Select “Trash” and click subscribe.
Do this for any other folders you wish to sync with the server.
Uncheck the option “When displaying hierarchy in Outlook, show only subscribed folders.” at the bottom of the window.
Click “OK” to close the window and commit the options.
This will resolve extremely slow synchronizations with Microsoft Outlook platforms. This issue can still be noticed if you have a very large Inbox folder, Sent folder, Trash folder, or any subscribed folder. The recommended method of avoiding this outlook issue is to simply file away messages from these subscribed folders into archived/unsubscribed folders (this also keeps your email organized!). Also, you can simply archive messages in outlook to clean them out of your mailbox (be careful as this WILL remove the messages from the server). Lastly, be sure to empty the Trash folder frequently enough to avoid a constantly growing Trash folder.
by Joseph Hanna.