Since I found out about Outlook's "Cached Exchange Mode" I've been using it, and I also set it up on every PC I come across. I think it's a great feature that improves the efficiency of of every Outlook user, and I don't understand why it's not ON by default.

My question is - are there any disadvantages to using Cached Exchange Mode? (Besides the obvious fact that it consumes a bit more space, which I don't see as a big issue nowadays)

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    Uuhhhh.... it IS on by default since Exchange 2003, unless you're using a customised installer package for Office/Outlook, or perhaps a Group Policy setting. – ThatGraemeGuy Oct 28 '09 at 21:11

10 Answers 10


If you only use one PC, none. If you work in a hot-desking environment, or even just occasionally use another PC, the your entire Exchange folder will be downloaded when you start Outlook. If you are subject to a quota, not such a problem but if you've several GBs of data stored on Exchange, bit of a pain in the network.

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    Our administrator publishes Outlook on a terminal server and doesn't need dozens of users keeping a copy of their overstuffed 2GB Exchange cache there. – Stephen Jennings May 1 '10 at 4:37

Coming from a Microsoft Active Directory (read as Microsoft Domain) environment I have seen cached exchange mode cause more issues then it "solves" and I generally recommend users turn it off.

Here are a few examples of issues that it can cause:

  1. Some files will be stored locally and not propagate to the exchange server as they should.
  2. Files that you want to share (mainly outlook calenders) will point to the local content and not to the server content, thus preventing the content from being shared.
  3. Tying in with the previous point permission relationships can also be broken making both the users, and administrators life harder with unneeded issues caused by cached exchange.
  4. It can also affect the response time to identify issues before they grow more severe. If the user comes to the admin and says that they can not send or receive mail the administrator might be able to isolate the issue (e.g. an ip conflict) or identify a more severe issue before it grows out of control (such as an improperly configured switch turning on port security blocking every user using the switch from connecting to the network... see as mass hysteria and every user thinking that the world is on fire despite the, maybe, two minutes it takes to fix the issue once identified). I like to see it as an early warning system for the sys admin / helpdesk / or unfortunate soul that got roped into doing IT even-though it's not their job / profession.
  5. Also if the OST (which is created when you enable cached exchange mode) becomes corrupt and has not synced all data with the server it WILL be lost. (The same applies if the user profile moves/changes and get disassociated with the OST created)

That's enough for examples and time for a suggestion.

If you want to store your email locally to view often I would back them up to a PST. With PSTs not only can you password protect your data (if you so wish) and but also keep it with you on a flash drive or the like (consistent backups are a must!). If your PST is growing close to 20 GB I would recommend making a second one to store more recent or a more specific set of data, e.g., funnycatpictures.pst, receiptsofclientorders.pst, emailsfromthatoneguywhoiwishwouldstopemailingmeonesenatnceresponses.pst as past 20 GB PSTs become unstable and are prone to corruption, unless you are using MS Office 2010 or later where the limit is 50 GB.


When Cached Exchange Mode is enabled, Search Folders will not work properly in Outlook Web Access. For example, you may not be able to see the Unread Mail search folder.

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    Welcome to superuser. Please do not include signatures or sign-offs in your answers, questions, or comments. – Jeremy W May 30 '12 at 4:30

I use Outlook in Cached Mode on several computers, and the main disadvantage is that changes are not propagated to the server immediately. This means that if I move a message to a folder, or add an appointment to my calendar and then immediately suspend, those changes aren't reflected on any other client (until I wake back up and give it a few minutes to sync).

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    How long is the delay? – Roee Adler Oct 28 '09 at 20:47

What about the obvious security issue about having OST files left on the users local PC? Anyone could access the OST files and get unauthorised access to their emails. For that reason alone it may be appropriate to disable cached mode.

  • Anyone having access to your computer can use Outlook and can gain access to all the email as well. Actually them having access to your computer would be a more serious problem than them having access to your OST file. – Jim Raynor Dec 12 '17 at 22:20

I suppose that you could be used to Exchange and think you are connected, click send and quickly suspend or shutdown before being on the VPN / network and then forget to send? :S

I can't really think of any serious downsides either.

  • New feature? Wasn't it introduced 6-7 years ago? :) – Roee Adler Oct 28 '09 at 19:13
  • ehh... been a while since I used Exchange last... I will delete last para! – William Hilsum Oct 28 '09 at 19:16

Not sure about newer versions but the default in Outlook 2003 was to only sync directory changes once a day in cached mode.

This means that you might not be able to email that new member of staff to welcome them to the office if they only started today, also you'll only pick up mailing list membership changes (and new mailing lists) at the sync, so if you send a mail to your team's mailing list welcoming the colleague who's just transferred in they might not get it.


This guy's saying that Exchange mode has these issues:

  • More work for the Exchange Server
  • Faster access to emails when hot desking or using terminal server, etc...



Also, it's harder to manage from an admin point of view.


From https://johanveldhuis.nl/outlook-cached-mode-gebruiken-of-niet/:

Scenarios where you don’t want to use Cached Exchange mode:

  • Computers which are being used by multiple people and where a delay of downloading the new messages is not acceptable;
  • Environments where compliance and security rules which prohibits the store of data locally;
  • The complete cached file can’t be stored because of lack due to disk space;
  • Mailboxes greater then 25 GB;
  • Virtual or Remote Desktop Service environments where Outlook 2007 and Outlook 2003 is installed. Cached Exchange Mode isn’t supported on a system which offers Remote Desktop Services;
  • Virtual or Remote Desktop Service environments where Outlook 2010 is used but not enough disk space or I/O’s are available;

The problem kicks in when the mailbox is over 2GB in size. Outlook hangs or crashes and you have to delete your profile and reload it all over, resulting in probably the same thing.

It's not cool to tell your CEO that his mail is gone for the next little while. Oh, you had .PST archives too? Where are they located? Oh, you don't know?

It's great if you inform your employees in time to archive their mail, save the data files on the file server, and everyone knows exactly what that means. Sometimes that message gets a little lost in translation.

I can never recommend anyone running cached-mode on workstations will never get dragged out of the building anyway. Laptops, possibly, but only if the person is informed not to stash up to 6 years of mail in their mailbox.

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