While the Mail folder does indeed contain all of the messages that are in the mail box, simply copying that folder into the new User Liberary will not restore the mail box. There is another file that is needed. Assuming that the path to the mail folder is:
Macintosh HD/Users/MyHome/Library/Mail
Then, if there are any downloads that are needed to be copied you need to also copy:
Macintosh HD/Users/MyHome/Library/Mail Downloads
And you certainly will need to copy:
Macintosh HD/Users/MyHome/Library/Preferences/com.apple.mail.plist
Of course you could simply copy the entirety of:
Macintosh HD/Users/MyHome/Library
to the new user home on the freshly reformated HD, but that would carry with it a lot of the dead wood you were trying to get rid of in the reformating process.
I just went through the same agony and even the CSR at AppleCare did not know about the .plist file. But it contains all of the user info and the pointers to the message data base and is absolutly necessary for Mail to read the messages.