Although you may have several folders open at once, only one folder can be active at a time. Actions such as searching, sorting, and deleting messages affect the active folder only. The Folder pop-up menu in the main window contains the names of open folders. To display the contents of an open folder in the main window, simply select it from the pop-up menu. If you want to open a folder read-only, push the Read Only toggle button in the folder dialog box before pushing the Open button.
Folders load much more quickly and consume far less system resources when they are opened read-only.
If you do not intend to update the folder you are opening, you should choose this option. Updating folders When you update a folder, changes you've made to it become permanent. Files that are marked for deletion are removed permanently. Files that have been saved to other folders are also deleted from the active folder when you update it. To update a folder, push the Update button at the bottom of the window, choose Update from the Folder pulldown menu or click on the update icon in the toolbar.