Where does Mail keep its configuration files?
Solution 1:
Mail.app stores server credentials in your login keychain. You will find your login keychain in: ~/Library/Keychains/login.keychain
Copy, rename, and open this keychain file with Keychain Access. Keychain Access is included with Mac OS X in the Applications > Utilities folder.
Within the Keychain Access application, search for your mail server using the search field at the top-right.
To retrieve a password, double-click on the item and enable Show password:.
Solution 2:
I found the file with the account list under
~/Library/Mail/V2/MailData/Accounts.plist
It seems to hold all sorts of metadata related to accounts but if you go through it you'll find things like server names and ports.