Microsoft created Cached Exchange Mode so that users can access an offline copy of their mailbox even when they're disconnected from the Exchange server or the server is down or unavailable. Although users can’t send and receive mail, they can compose messages for future delivery, and they can read all messages that were copied to the cache prior to being disconnected. Sometimes, Cached Exchange Mode might be your only line of defense against system failures.

Any good administrator will tell you that you should always back up your system, test your backups, and have enough redundancy in place to prevent catastrophic failures from disrupting mail flow. In the real world though, these best practices aren’t always followed. Budget cuts and lack of training sometimes lead to subpar Exchange Server deployments.

I once received a call from a client whose Exchange server had catastrophically failed. I discovered that the hard disk containing the database had died, and it was in such bad shape that the data was unrecoverable. The Exchange server had never been backed up.

I was able to fix the problem by going to each individual workstation and copying each user's cached data to a PST file. I then reinstalled Exchange and created new mailboxes for everyone. I attached Outlook to the new mailboxes, then moved the contents of the various PST files into the users' mailboxes.

Obviously, this technique wasn’t an ideal recovery solution. It was tedious and time consuming, to say the least. But had Cached Exchange Mode not been enabled, the company would have lost all of its mail server’s data. As ugly as my recovery solution was, it gave me a shot at rebuilding the server.

Incidentally, if you ever try using this technique, don't be tempted to skip the step of copying data to and from PST files. The .ost file in which each user's cached data is stored is linked to that user's Exchange mailbox. If you try to recreate the user's mailbox, and then link Outlook to it, the cache will become invalid, even if the new mailbox has the same name as the old one.