Details
Description
Since CiviCRM 4.5 and the change from a HTML multiselect to a javascript autocomplete, it's now possible to select groups that are not of the type Mailing List. It could lead to problem like people not able to unsubscribe for example.
To reproduce, select Mailings -> New mailing and in the first page, you can select the group "Administrators" which is not a Mailing List
Seems to be fixed in CiviCRM 4.6 with the new UI.