Undeliverable mail message when booking a meeting

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Hi,

We are using exchange server 2003 on a windows 2003 server, all clients are accessing their emails via Outlook.

When sending a meeting request to user1 we get an undeliverable email message about another user (lets call him user2) that left the company.

I checked the outlook option for user1 and there is no delegates. User2 has left the company, his account and mailbox were deleted. There are no records of user2 anywhere in AD or in Exchange Manager. How can i get rid of that undeliverable mail message?

Thanks