Here is the scenario: User (me) has Office 365 Exchange (P plan). User has 10GB of mailbox data. User has desktop, laptop, and Surface Pro. User wants full access to entire Exchange repository on all devices using native Outlook - not web interface. User sets up Outlook to connect to Office 365 Exchange using cached mode on all devices. This setup does work and achieves user objectives. [One aside: user uses tasks extensively and needs full Outlook, not Windows 8 Mail App, to access tasks].

However, user runs out of space on C:\ drive on desktop. User moves .ost files to D:\ drive. User creates symlink in standard location to point to .ost on D. This works on desktop. User then discovers that the laptop and Surface can no longer connect to Exchange because the .ost file at c:\users\username\AppData\Local\Microsoft\Outlook\ is not valid.

My current thought is to replace my 120GB SSD on my desktop with a 240GB C:\ drive and return the .ost files to their original location.

Is this the best solution?

The implication and side effect will be that all my devices that carry full cached .ost files will need to allow up to 10GB (and up to 25GB or more in future as my repository grows) on the C drive for .ost files.

A large Exchange mailbox repository is important to me but this is not elegant or pleasing. Is there a good alternative? Am I missing something?



Update to issue:

It turns out that the failure of the other instances of Outlook 2013 to connect to Exchange only occurs if the computer with the symlinked .ost file is also connected. If Outlook is closed on that computer the other computers can then connect.

The page entitled "Choose between Cached Exchange Mode and Online Mode for Outlook 2013" on TechNet at technet.microsoft.com/en-us/library/jj683103.aspx says that a scenario to use Online Mode instead of cached mode is when you have:

  • Very large mailboxes on computers that have insufficient hard disk space for a local copy of the mailbox.

A way to interpret that would be to say: If I really want Cached Exchange Mode I'd better have enough space on C:\ to handle the storage.

So, it seems that there is a narrow issue: the symlink solution to moving the .ost file on a single machine from the C:\ drive to another drive has the side effect of interfering with the functioning of other instances of Outlook 2013 on other computers that try to connect to the same Exchange mailbox at the same time as the computer with the symlinks.

In principle Microsoft should treat this as a bug and fix it in the future.

  • How is that specific to Office 365? You seriously think that the outlook client you install is different than that of a company running their own exchange servers? – TomTom Jun 20 '13 at 15:59
  • Thank you TomTom. You are correct, the problem is not Office 365 specific. – Michael Cullina Jun 21 '13 at 11:25
  • Retagged - ms-office-265 replaced with ms-office. – TomTom Jun 21 '13 at 15:13

This is a great question, since we recently migrated to Office 365 from an on-prem Exchange server.

For us, what caused us to question this is the 49.5 GB of mailbox space -- and the desire to get rid of offline archives. I mean, really, if you don't need to archive, do you really want a cached mailbox that's nearly 50 GB in size? Heck no!

Not to mention whatever you archive is outside the control of IT. If a user loses it, and it hasn't been backed up, it's GONE.

For us, we also wanted the flexibility to be able to have all the laptop users access some cached data so they can easily work while on a plane or otherwise offline.

Our answer applies to Outlook 2013 users:

When setting up the user account in Outlook 2013, you get to choose how much data you want to cache based on a timeline. In the Server Settings =>> Offline Settings select "Use Cached Exchange Mode" and pick how many months (up to six in our case) that you want to cache locally. For us, the magic number was two months.

One extra benefit with Outlook 2013: the cached file is said to be 40% smaller than the cached files created in earlier versions. Now I can have a 50 GB mailbox, avoid the pain of missing archive files -- and still offer the convenience for travelling workers to be able to work offline if they want to. I say that's a good deal!

BTW, if they're not showing the image in this post, try going to

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It honestly depends on what is being cached. Many mobile devices will cache the header and the body will be downloaded once the user opens the message.

To answer your question - the laptop having cached exchange mode will download all the email on the exchange server to an OST file on the local machine. Since you have a laptop a desktop and a surface pro running a full blown version of outlook this hold true across the board. You can change the location of the OST in the settings as well as the registry. Microsoft allows this since Office 2007. Prior to that the location wasnt easily changeable. I believe you are also able to change what is cached in outlook preferences.

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