blog-cached-online-mode-exchange_sqVarious configuration scenarios fit companies differently. Whether your Outlook users will be using cached or online mode depends on a wide range of factors.

Companies using Exchange Server are often confused around whether they should run cached mode or online mode on their Outlook clients. In this post, we are going to look at just what the differences are, and make some recommendations on when each is appropriate.

What is Cached Mode?

Cached mode, which first came out with Microsoft Outlook 2003, keeps a local copy of the user’s mailbox stored on the hard drive as an OST file. Running in cached mode, the Outlook client looks to the local OST file for all access, including reads and searches, while a separate process checks for new mail on the server and syncs data to the local cache.

Cached mode clients also keep a local copy of the GAL, called the Offline Global Address Book, to perform faster lookups for recipients in the organization. Running in cached mode, a user can still access mail even when the network connection to Exchange is down, such as when they are on an airplane or the WAN is down.

Cached mode is also very good for users with high latency connections to Exchange, as accessing the local cache isolates the user from delays in connectivity to the server. Searches done against the local OST are much faster, and the client generates much less network traffic.

You should use cached mode anytime a user must access their mail without network connectivity, such as users who travel. You should also use it for any users in an office with intermittent network connectivity, or whose network latency between client and server is typically high, like those with satellite or radio-based connectivity.

How is Online Mode different?

Online mode maintains a connection to the Exchange CAS server for all access to the mailbox and reads from the GAL. If the connection to the server drops, Outlook is unusable until the connection is restored. It also requires a much better connection to the server, as far as latency is concerned. The biggest difference though is that online mode does not require any disk space for a local file, making it ideal for clients with limited or no persistent storage.

You should user online mode when users have no persistent storage to which they can store their OST, such as VDI scenarios or on devices with limited storage capacity, such as tablets. You may also use online mode for extremely large mailboxes to improve overall performance, or where you do not want to risk having a local copy of the OST for compliance or other reasons.

I disagree with the general opinions about risks associated with using OST files. If you have a machine that leaves the physical security of your four walls, then you have a data risk no matter whether you use cached mode or online mode. Don’t fear the OST; instead, embrace full disk encryption such as that offered by BitLocker or various third party vendors, and ensure strong authentication is required to gain local access to the disk. It’s far better to secure all the data, then to shoot yourself in the foot with regards to Outlook performance.

Watch out for latency issues

Latency is really the most significant thing to consider when deciding between cached an online mode. If you have high latency, you should use cached mode; if you have consistent and reliably low latency, online mode is okay. What is high and what is low? That’s a good question which is as much answered by opinion as it is fact. As a rule of thumb, I consider 100 milliseconds to be the maximum latency for online mode.

If you are seeing client connections to the CAS server go over that consistently, you might want to switch to cached mode. Various Microsoft documents will vary between that and up to 500 milliseconds, and your own users’ experiences will ultimately decide what is good enough, but with anything over 100 milliseconds Outlook starts to pop notification bubbles that it has lost connectivity to Exchange. That usually generates helpdesk calls, and nobody likes those.


Cached mode needs local storage and that’s a problem for tablets with small SSDs or VDI systems without persistent storage. In the former, you can use a GPO to restrict the maximum size of the OST, as you may not need to cache the entire mailbox. When it detects a smaller disk drive, Outlook 2013 and later will automatically reduce the OST size by reducing the number of days cached.

For VDI, where persistent storage can be extremely expensive and sometimes even counter to the design intent, you have to make a tradeoff between space taken and performance. You can provision a certain minimum amount of persistent storage on high performance disks for VDI systems to store the OST, or instead you can direct VDI users to use OWA when latency is too high for online mode. If latency stays below 100 milliseconds, use online mode so you don’t have to provision persistent storage.

Which one should I choose?

For Office 365 or other hosted Exchange customers, the answer is easy – use cached mode. The Exchange CAS server is not local to you, so you will have higher latency and cached mode will accommodate this. With Office 365 in particular, Microsoft will not prevent you from using online mode, but if performance is poor and you call support, they will instruct you to use cached mode.

For the rest of you, consider the following – if you do any of these, online mode may provide the better experience for your users:

  • Delegating access, when folders are not cached locally due to storage constraints (and local cache is the default)
  • Opening another user’s calendar or folder that is not cached locally (local cache is the default)
  • Using a public folder that is not cached. Though really, you’re still using Public Folders?
  • Using one or more large (>1GB) shared mailboxes

Ultimately, I tell all my customers to use cached mode, and if they cannot for any reason, to use OWA instead of Outlook. This generally provides Outlook users the best experience overall, while making sure VDI admins don’t break the bank provisioning persistent storage for their users.

If you need to deploy settings for cached or online mode to your users through GPO, see for more guidance on that.

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