Hello Rebecca, you can enable automatic synchronisation for the folders by enabling the option “Download messages for offline use”.
If you’d like to use this option on all your subfolders regarding this account, navigate to Tools > Accounts > Your account > IMAP (Mail) tab and enable the download messages for offline use option for your account. In case you’d like to enable this for certain folders only, please right click the folder in the left pane and navigate to the folder properties where you should be able to enable offline options as well.
Hope this helps,