Such issue are often caused by slow internet connection or very large IMAP sub-folders and email inside those sub-folder
As IMAP is a very bandwidth intensive protocol, we recommend that you sync your Inbox on send and receive and disable auto-sync for the rest of the folders.
- Launch your Outlook
- Go to Send / Receive Tab > Send/Receive Groups > Define Send/Receive Groups
- Select All Accounts under Group Name and click on Edit
- Select your Email Account
- Do the following action
- Uncheck Get folder unread count for subscribed folders
- Check Use the custom behabior defined below
- Check Inbox and untick the rest of the folder
