At present Outlook 2003 is supported with Exchange Online as the back-end platform is Exchange Server 2007.
The main caveat when using Outlook 2003 with Exchange Online is the lack of Free/Busy functionality. This simply doesn’t work with Exchange Online out of the box. One option is for users to switch to Outlook Web Access (OWA) to perform any calendar functions that may require Free/Busy information.
The recommended option however is to use the Outlook 2003 Connector (link available on our downloads page) – which builds that Free/Busy functionality back in.
Going forward however, Outlook 2003 will not be supported when Wave 14 (Exchange 2010) is rolled out to Exchange Online.
Customers will have 12 months from Wave 14 release to perform the Exchange Online upgrade – which gives them opportunity to update their version of Outlook.
Something that will assist in this decision is the fact that mainstream support of Outlook 2003 ended on 14/04/2009, and it is now in the Extended Support phase.