In previous versions of SharePoint, Outlook was simply a handy tool for storing local copies of specific types of SharePoint lists, such as contacts, tasks, and calendar lists. It was great for offline access to these types of SharePoint lists, but it did not allow the user to update them locally and then replicate that modification back to the SharePoint server. But the new Outlook 2007 is very different. It is almost as if this version of Outlook is acting as a SharePoint client, besides being a mail client. It brings so many enhancements to SharePoint users, and for that reason, I recommend that even if, for some reason, you choose not to implement Office 2007, you implement Outlook 2007!
It is definitely possible to use MS Outlook 2003 with SharePoint 2007. However, it will not have all the new and exciting features that Outlook 2007 offers. Below is list of the standard features that Outlook 2003 offers:
Task lists: Outlook allows the user to download a copy of tasks lists for offline usage. These tasks are read-only and changes cannot be replicated back to the SharePoint server, not even with SharePoint 2007.
Contact lists: The same thing is true for contact lists. The user can download a read-only copy of a SharePoint contact list, but it cannot be changed in Outlook 2003.
Calendar lists: Again, same thing is true; you can only download a read-only copy of SharePoint's calendar lists.
Alerts: With Outlook 2003 you can receive and use ...