I have set up a second copy of Exchange on a separate server. I have a hardware equalizer with a heartbeat, so if the first Exchange server goes down, the traffic will switch to the second server. What do I have to do to mirror/synchronize mailboxes to the second server, and how do I set up my MX records accordingly?
Unfortunately, there are no native Microsoft technologies to accomplish this. Depending on the distance between your servers and the specifics of the technologies you're using, you may find that a geo-cluster solution (i.e., NSI Double-take) or something like Marathon Technologies will meet your requirements. Or, simply explore active/passive clustering for Exchange. Have a read through
Exchange Server 2003 cluster architecture
to learn more.
How you set up your DNS will depend on which approach you take, whether you implement DNS load balancing (i.e., BigIP, round robin DNS, etc.) and what kind of firewall you're using.
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Related information from the TechTarget Windows Network:
SearchExchange.com Interview: Scott Schnoll Q&A: Clustering Exchange 2000
SearchExchange.com Reference Center: Replication and Synchronization