Current Status: issue with mitigation
Unlike outlook 2007 and 2010, Outlook 2003 clients rely on public folders. If a public folder database doesn't exist, Outlook 2003 users will be blocked from connecting to their exchange 2010 mailbox and receive the error message shown in Figure 8.
Figure 8: Error message when an Outlook 2003 user connects to an exchange 2010 mailbox
There are several reasons why a public folder database is required for Outlook 2003 client. First, Outlook 2003 in cached mode uses the “OFFLINE ADDRESS BOOK” system folder to download the offline address book (OAB) and the “SCHEDULE+ free BUSY” to retrieve and update free/busy information.
Figure 9: Offline Address Book and Schedule+ Free Busy system folders
Second, if you're installing Exchange 2010 into an existing Exchange organization running exchange 2007, it's important you add the Exchange 2010 public folder database to the replica list of the “SCHEDULE+ FREE BUSY” folder. If this step isn't completed, users who use Outlook 2003 cannot publish their free/busy data in Exchange server 2010. Instead hash marks appear in the free/busy data for these users. More information as well as the steps that can be used to remediate this issue can be found in the following KB article:
Special Thanks to Henrik Walther