Although you can restore messages from a Time Machine snapshot within the Mail application, it generally won't work with messages that were saved by an obsolete version of Mail. In that case, you have to use an alternative method.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail
Right-click or control-click the highlighted line and select
Services ▹ Open
from the contextual menu.* A folder should open.
Enter Time Machine and scroll back to the snapshot you want to restore from. Inside the folder named "Mail" are subfolders named "V2" and/or "V3" (depending on the version of OS X that was in use at the time the snapshot was taken.) Inside one of those subfolders are more subfolders representing your Mail accounts. The names refer to the email addresses you use. Select the account folders you want and then select Restore ... to... from the action menu (gear icon) in the toolbar of the snapshot window. Restore the folders to the Desktop, not to their original location.
From the Mail menu bar, select
File ▹ Import Mailboxes...
Choose Apple Mail as the format and import from the mailboxes in the folders you restored to the Desktop. The imported messages will appear in a new mailbox. Move the ones you want to keep wherever you like and delete the rest. Then delete the folders on the Desktop.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
