…is just as easy but a little different. The change is effectively the same but the plist file is in a different location.

It should be in:

~/Library/Mail/V2/MailData/Accounts.plist

The key location should be the same, although in Lion Xcode the dictionary items are named “Item 0”, “Item 1”, etc., instead of 0, 1, 2, etc..

Note that if you upgraded from a previous version of OSX, you’ll still have the old plist file and it will still have a MailAccounts setting, but making changes there won’t do anything.