During an Exchange 2003 to 2010 transition a legacy namespace is configured for users who are connecting to Exchange 2003 mailboxes, and the certificate must be imported on the Exchange 2003 server.
Exchange Server 2010 like its predecessor Exchange Server 2007 makes heavy use of SSL certificates for various communications protocols. Before putting a new server into production you should create and assign a new SSL certificate for the server.
Your Exchange server will run for 120 days without a product key but you should enter one as soon as possible and certainly before placing the server into production.
A Typical Exchange Server 2010 is one that runs the Client Access, Hub Transport, and Mailbox server roles. This is the type of installation that would be performed in a single server organization.
How to update the schema for Exchange Server 2007 SP2 and Exchange Server 2010 prior to deploying Exchange.