It is possible to mix data from two data sources, at least in theory. You will have to come up with a mechanism to associate appointments with the data source it came from, but this can be easily achieved by marking them with a custom attribute. You will also need to ensure that their IDs do not collide.
Exchange 2003 is not supported, as EWS (exchange web services) are available from Exchange 2007 and up. Unfortunately, we do not support the legacy interoperability mechanisms.
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