Flag/Unflag shortcut in mail.app

You can copy the folder "Mail" in the folder "Library" in your Home Folder (~/Library/Mail) and put it back in the same place whenever you want.

and according to Apple Support Center:

*  To back up Mail preferences and messages:

  Note: These steps back up Mail preferences, but do not back up Mail messages (emails), although you can do that with the .Mac Backup utility. See below if you want to manually back up Mail messages (mailboxes).
     1. Create a uniquely-named folder on your desktop (or other favorite location). For example, you might call it "Mail Prefs Backups May_15_2005" (adding a date is a good idea).
     2. Quit Mail if it's running.
     3. In the Finder choose Home from the Go menu.
     4. Open the Library folder.
     5. Open the Preferences folder.
     6. Copy the "com.apple.mail.plist" file to the folder you made in step 1 by Option-dragging the file. Don't remove the original file from its current location (this file contains your Mail accounts information).
     7. Close the Preferences folder.
     8. Open the Mail folder.
     9. Copy the "MessageRules.plist" file to the folder you made in step 1 by Option-dragging the file (this file contains your Mail rules).
    10. Copy the "SmartMailboxes.plist" file to the folder you made in step 1 by Option-dragging the file (you guessed it, this file contains Smart Mailboxes information).
    11. Finally, copy the Signatures folder to the folder you made in step 1 by Option-dragging the folder. (Your custom signatures, if you use them, are stored here.)
* To restore:

  Move all copies of the files and folders from your uniquely-named folder to the original locations from which they came, as listed above.

Most likely because your corporate exchange server is running Microsoft Exchange Server 2003. Snow Leopard now only connects to Exchange 2007 (See http://www.apple.com/macosx/exchange/) whereas Leopard used to connect to Exchange 2003 using the OWA mechanism.

iPhone supports both Exchange 2003, 2007 and 2010 it seems (See iPhone User Guide , page 2);