If the message you want to view is in the default Folder directory:
Press MENU on the abbreviated menu button to the left of the Mail File text field.
A menu pops up listing the messages in the Folder directory.
Drag the pointer to the message you want to load, highlight it, and release MENU.
All messages saved to that file appear in the Mail Tool base window as separate messages, just as they did when first received in the In-Box.