What does this really mean?
You have 2 servers that act as one. Using common examples, you would setup a server in one data center and a VM server in another data center. We normally set both machines up in DNS with the same hostname (mail.yourdomain.com) for MX purposes so that incoming mail is load balanced between the 2 servers. Each server also receives a unique hostname so that you may use either server for client access and SMTP services. Emails coming in to your email address may be delivered to either server.
Since both servers are load balanced and incoming mail is received by either server, virus and spam scanning is distributed between the two servers effectively doubling your incoming mail capacity.
Then you may either have your users use mail.yourdomain.com to access their POP3, IMAP, and SMTP services or dedicate one machine for outgoing mail and one for POP3 access. If you set your clients to use mail.yourdomain.com they will automatically be load balanced between the two servers. If they receive a message on server-a but are logged into server-b, the message will automatically be replicated and appear in their mailbox on server-b as well. If they change their password on server-b and then attempt to login to server-a, their new password will automatically have been replicated to server-a.
This effectively gives you 100% uptime since the possibility of 2 servers that are in separate data centers both being offline at the same time for whatever reason is astronomical. If one server is offline, the other server will automatically accept mail for your users.

Finally - redundant email for you, your business, or your clients.


We have created a 3-part how-to video on how to set this up yourself. The video series is available for $69.95 here.