Cached Exchange Mode, a must for mobile devices using Exchange?


Our "Hosted Exchange 101" post got a fair amount of traffic and commentary, so I thought I’d follow up with a related topic: Cached Exchange Mode. Just like the last article, this isn’t meant to be a detailed technical commentary but to quickly hit a useful topic. This concept applies to any Microsoft Exchange account, either hosted or internally served, but of course your I.T. department should take precedence over anything I relate here.

Cached Exchange Mode, what is it? This mode keeps a local copy of your mailbox on your device, in the form of an .ost file also known as an offline storage file. In this mode, there are two full copies of your mailbox: one on the Exchange server and one on your local device. When you’re offline, you can make changes to the local copy of your account and those changes will be automatically synchronized with the server when a connection permits.

Why would you want to use Cached Exchange Mode?
There’s both an advantage and a disadvantage to CEM. Let’s start with the advantage: if you’re mobile you may not always have connectivity or perhaps you don’t want to use your battery juice with a WiFi or modem connection. Without CEM, how would you access your Outlook data? Having a local copy of your mail, contacts, appointments and such would keep you in business without the connectivity.

That’s the upside, so what’s the downside? Having a local copy obviously takes up space on your hard drive. For mainstream notebooks, that’s unlikely to be an issue, but many UMPCs come standard with 30- or 40 GB hard drives. You’ll have to balance out your needs from a connectivity vs. storage capacity depending on your particular configuration.

Optimizations: In this mode, you can configure Outlook to be optimized for your connection type based on these options:

  • Download full items
  • Download headers and then full items
  • Download headers only
  • On slow connections, download headers only (which automatically senses the connection speed and optimizes available bandwidth for minimal information)


I personally use Cached Exchange Mode with my hosted Exchange service on every mobile computer I own. Proper Inbox maintenance combined with routine archiving are essential to negate the hard drive capacity challenge, but it’s well worth having access to my data locally. If you want to enable Cached Exchange Mode, just check your e-mail account settings and look for the option. It may take some time to set up your initial Inbox as Outlook has to pull down a copy of your account from the Exchange server, but once you’re set up, the process is seamless and quick.