Like Windows 2000 and Active Directory, Exchange 2000 also has native and mixed modes of operation. Moving your Exchange organization to native mode offers advantages over mixed mode, but you must thoroughly understand the differences between native and mixed mode before planning a switch to native mode.
By default, Exchange 2000 installs and operates in mixed mode. Mixed mode allows Exchange 2000 and Exchange 5.5 servers to coexist and communicate. However, this backward compatibility limits administrative flexibility. Under mixed mode, Exchange 5.5 sites map directly to administrative groups and administrative groups map directly to Exchange 5.5 sites. All servers in a site must use a common service account, just as with Exchange 5.5. In addition, routing groups only contain servers from a single administrative group.
Native mode allows more flexibility than mixed mode. With Exchange in native mode, you can place servers from multiple administrative groups into a single routing group, and you can move servers between routing groups. You can do away with the requirement that all servers in a site must use a common service account. Additionally, operating in native mode allows you to move mailboxes between servers in the organization (removing the intersite mailbox move limitation in Exchange 5.5). For some companies, this enhanced mailbox move capability is reason enough to switch to native mode.
Your Exchange organization is a candidate for native mode operation if you have no remaining Exchange 5.5 servers—or plans to add any—and you don't require Exchange 5.5 connectors.
Now that you know about native vs. mixed mode, you may want to start planning a switch to native mode. While making the switch isn't difficult, it's permanent. Begin testing and refining your plan for switching to native mode in a lab environment now.