You may to wish to back up your mailboxes, messages, addresses, and account settings as part of your regular backup strategy. To back up this information, make copies of the files listed below.

Copy all the files located in ~/Library/Mail.
Copy the file “com.apple.mail.plist,” located in ~/Library/Preferences.
This file stores the preferences you set in Mail Preferences.

Copy the folder “AddressBook,” located in ~/Library/Application Support. This file is used by both Address Book and Mail.