1. You haven't specified the version of your Mibew Messenger.
2. Presuming that you're using the latest Mibew Messenger (i.e. v. 2.2.2), have you properly set email addresses for the groups?
3. If so, have you checked the mail section of your configuration? What's there?
4. Have you checked the error logs of your web server?
5. Have you checked the logs of the mail server?
The more information you provide, the higher possibility that you'll get a solution.