* 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". Design: This should only work from a domain that exists in postfixadmin to a domain that already exists in postfixadmin. If the target domain does not exist, we should error out as soon as possible. Once we're sure that the target domain does exist, we can hand off the "mv" operation to the plugins. Some of them might have nothing to do -- that's fine. * Add convenience methods (e.g. user_exists, domain_exists) that we can use in the tests to check results. * Potentially add OpenDKIM support.