You need to copy and then replace the ~/Library/Mail folder (where "~" is your username). So, the Library/Mail folder in your Home directory. Copying that folder in to your Panther install will give you the same Mail as you had on Jag.
I don't think that the preference file in ~/Library/Preferences is necessary (I've never copied it), but it is in there as com.apple.mail.plist (or something similar)...
And don't forget your Address Book! Same deal with ~/Library/Application Support/AddressBook/ -- copy the whole folder.
good luck