Backup Mail Preferences

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).
Quit Mail if it's running.
In the Finder choose Home from the Go menu.
Open the Library folder.
Open the Preferences folder.
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).
Close the Preferences folder.
Open the Mail folder.
Copy the "MessageRules.plist" file to the folder you made in step 1 by Option-dragging the file (this file contains your Mail rules).
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).
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.)