From Secure Computing Wiki
I found a great little how-to over at http://www.procata.com/ explaining what you need to transfer all my email and preferences for mail.app. This is my own wording, but I give credit to those guys.
- First, copy the entirety of /Users/<username>/Library/Mail to the new user's matching directory:
sudo cp -r /Users/<old_user>/Library/Mail /Users/<new_user>/Library/Mail
Second, copy the Mail.app preference file:
sudo cp -r /Users/<old_user>/Library/Preferences/com.apple.Mail /Users/<new_user>/Library/Preferences/com.apple.Mail
- Note, the file name may be com.apple.mail on more recent versions of OS X. Unix systems are CaSe SeNsItIvE.