Unable to send e-mail. Please contact the site administrator if the problem persists.

Is there any other information? It is a little difficult for anyone to help you with that. Do you have any error messages in your Drupal logs, or your web server's logs?

Hey, I just had the same problem as you. My problem was, and I don't know if you solved this yet, is that the group I was attempting to send an email to had no members, because after I created the group, I forgot to add the roles to the users. Just a thought.