I have the same pb. wich was reported before please give us a solution :
We are running release 6.0.1 on our servers. This issue is still occurring for us where a user is able to send a message to an Access Control List Only group. If a user selects the group by selecting the “Address” option from the tool bar, a message does display saying, “The selected entry does not have a valid address” however, it appears that when using type-ahead to address a message, the user does not get this message and is able to send the message to the ACL only group. Are there any plans to change/fix how this works?