You can upgrade directly to Exchange Server 2010 from Exchange Server 2003 or Exchange Server 2007.
During coexistence of two separate messaging systems, you need to consider access to public folder contents, e-mail message flow, global address list synchronization, and calendar information.
There is a number of Exchange Server 2003 and Exchange Server 2007 features that are not supported in Exchange Server 2010. You need to consider how to replace them.
Upgrade plan from Exchange Server 2003 to Exchange Server 2010
Before upgrading from Exchange Server 2003 to Exchange Server 2010, you must ensure that your Exchange organization is in native mode, update to Exchange 2003 Sp2 and that domain and functional forest levels are at least Windows Server 2003.
During coexistence, an Exchange Server 2010 Client Access server can be used to provide Outlook Web Access to Exchange Server 2003 clients.
All Exchange Server 2010 servers are part of a single routing group to support compatibility with Exchange Server 2003.
An Exchange Server 2003 administrative design cannot be directly replicated for Exchange Server 2010 because Exchange Server 2010 uses RBAC to define administrative permissions.
You can remove Exchange Server 2003 back-end servers after all mailboxes are removed from each server.
To simplify message routing during coexistence, you should completely upgrade one location at a time.
Upgrade plan from Exchange Server 2007 to Exchange Server 2010
To coexist with Exchange Server 2010, you must update Exchange Server 2007 to SP2.
An Exchange Server 2010 Client Access server will direct requests for an Exchange Server 2007 mailbox to an Exchange Server 2007 Client Access server.
Message transport coexistence is configured automatically because Exchange Server 2007 and Exchange Server 2010 both use Active Directory sites to calculate routing.
When you are assigning permissions on Exchange Server 2007 servers, use the Exchange Server 2007 groups. When assigning permissions on the Exchange Server 2010 servers, use the Exchange Server 2010 role groups.
As mailboxes are moved from Exchange Server 2007 to Exchange Server 2010, you can remove Exchange Server 2007 Mailbox servers that no longer have mailboxes. Hub Transport and Client Access servers should be removed only after all Mailbox servers in a site have been removed.
When selecting sites to upgrade first, start with Internet accessible sites.
Designing Exchange 2010 Coexistence with Other Messaging Systems
Exchange Server 2010 can be configured for coexistence with other Exchange Server organizations, other SMTP messaging systems, and non-SMTP messaging systems.
When integration with another Exchange Server organization, you need to consider the namespace, GAL synchronization, and free/busy information.
If you use unique SMTP namespaces for each messaging system, the e-mail address for a user changes when a user’s mailbox is moved between the two messaging systems.
You can use a single namespace for two messaging organizations. The second messaging organization could be an Exchange Server organization or a different SMTP messaging system.
By synchronizing GALs, you can ensure that all recipients are available for addressing in both organizations. Consider automating GAL synchronization.
By implementing calendar sharing between messaging systems, you can allow users to view the schedule of users in the other organization when sending meeting requests. Typically, you configure calendar interoperability only between Exchange Server organizations.