When you first synchronize your IMAP account, all the messages in all your folders will be downloaded unless you hide some of them or change their synchronization settings, for example, from All Messages to Headers Only.
Your mail folders will be displayed in the main window.
In the Show/Hide IMAP Folders dialog box, select a folder, and click Hide to prevent downloading its contents. Click Show to view the folder and download its contents.
Only the displayed folders will be downloaded when you synchronize the account.