If you have a small Notes server to migrate, you can choose to stop using Notes and start using Microsoft Exchange Server at migration time. For larger domains, you may need to plan a phased migration where both Notes and Microsoft Exchange Server are coexisting.
During a phased migration, you use the Microsoft Exchange Connector for Lotus Notes for message transfer and directory synchronization between a Microsoft Exchange Server organization and a Lotus Notes domain. For more information about using the Microsoft Exchange Connector for Lotus Notes, see Microsoft Exchange Server Operations and Microsoft Exchange Server Concepts and Planning in the Microsoft Exchange Server 5.5 documentation.