Outlook tracking tab shows attendees as optional when initial invite was sent to alias

This is issue is seen in the following scenario:

Outlook 2007 users with mailbox in exchange 2003 or 2007

User proposes a  meeting and has as only attendee an alias. After members of the alias accept the meeting, the meeting tracking tab shows those attendees as optional attendees.

Outlook meeting requests: Essential do’s and don’ts

http://office.microsoft.com/en-us/help/HA011276781033.aspx

It is not recommended practices to send meeting request to a Distribution Group. If you need to invite all the members of a distribution list, expand the list in the To line before sending the request.