* Error reporting sucks, and when a domain or user doesn't exist we should be able to say so. The describe_domain/user functions should also work better. For plugins that don't implement domains, we can return a (count of?) list of users, or fall back to the user deletion descriptions. * Implement "mv". * Add convenience methods (e.g. user_exists, domain_exists) that we can use in the tests to check results. * Potentially add OpenDKIM support.