Or your exchange server?
Assuming by group email you mean a distribution group and assuming your using an exchange server you might need to check the authentication settings of the distribution group.
In Exchange 2007 (not sure if it's the same in Exchange 2010) look under the properties of the distribution group in the 'mail flow settings' tab get the properties of 'Message Delivery Restrictions'. On this screen there is a tickbox option labelled 'Require that all senders are authenticated'. Make sure it's unticked.
Basically it only allows Outlook clients to send to that particular distribution group.
Thanks very much, that was helpful