Exchange 2003

I have a user who uses Outlook 2003 and resides on an Exchage 2003 server. Whenever this user sends a meeting request to a single user, the meeting request gets to the recipient. hen the recipient sends a response back, the recipient gets an undeliverable message back saying it could reach some or all of the recipients. The strange thing is the undeliverable message is saying it cannot reach an exchange group “all (companyname) staff” but this group is not in the To, CC or BCC field. Has anyone seen this behaviour before. There are no rules setup on the server or locally and no delegate access is setup on the account.

Thanks in advance