Calendar 2000 & 2007 Status of Meeting Attendees Changing

I have a client who has set up a meeting and sent an update and the following
occurs: "once I sent an update, my attendees changed to “optional� from
“required� and the tracking is inaccurate as someone who accepted is not
showing up as receiving a response. Another attendee did not receive the
update at all."

She also is asking whether or not it makes a difference is she uses a
distribution list in her meeting invitation.

Can anyhone shed any light on why this is happening?
--
LPS