A Microsoft Exchange Server 2003 organization and a Microsoft Exchange Server 2007 organization exchange communications by using SMTP. An Exchange 2003 user organizes a meeting and then sends a meeting request to an Exchange 2007 user. The Exchange 2007 user accepts the meeting request. The meeting organizer uses Microsoft Office Outlook to send a meeting update message or a meeting cancellation message to the Exchange 2007 user.
In this scenario, the meeting update message or the meeting cancellation message is not delivered to the Exchange 2007 user. Instead, the meeting update message or the meeting cancellation message goes into the SMTP retry queue. If an administrator tries to open the message in the Exchange System Manager console, the administrator may receive the following error message:
Unable to open for delivery