Thanks Jay for checking this out!
I guess I need to update my clients to Outlook 2003/2007.
Outlook and Exchange Server Compatibility
I also found this on TechNet website:
The following table lists the requirements for client computers that access data from Exchange 2007 servers.
Client application Requirement
Outlook
Use one of the following:
* Office Outlook 2007
* Outlook 2003
* Outlook 2002
Outlook Web Access
Client computer with a Web browser installed
Outlook Web Access using Integrated Windows authentication
Client computer with Microsoft Internet Explorer installed
Mobile device
Use one of the following:
* Mobile phones running Windows Mobile 6.1 or later versions
* Mobile phones running Windows Mobile 6.0 or later versions
* Mobile phones running Windows Mobile 5.0
* Mobile phones running Windows Mobile 5.0 with the Messaging and Security Feature Pack (MSFP)
* Mobile phones running a non-Windows operating system that are compatible with Exchange ActiveSync
Client application that uses Post Office Protocol 3 (POP3)
You must have a client computer that meets the requirements for your specific client. For a list of these requirements, consult the documentation for the POP3 client software.
Client application that uses Internet Message Access Protocol 4 (IMAP4)
You must have a client computer that meets the requirements for your specific client. For a list of these requirements, consult the documentation for the IMAP4 client software.
Microsoft Entourage
Download and read the Microsoft Office 2004 for Mac Resource Kit version 2. This resource kit contains a complete feature matrix and provides details about the versions of Entourage that are supported for Exchange 2007. Information about Entourage 2008 for Mac is available in the Microsoft Office 2008 for Mac Administrator’s Guide.