I am looking for a solution which would allow to use a convenience of Outlook as a mail client app while at the same time have no PST or OST files on a local computer.

Even in 'non-caching' mode Outlook creates an OST file where it downloads everything from the Exchange server. OWA does not create any local files (except cookies I believe) but lacks some of the nice features Outlook has.

Would it be feasible to place OST files on a network share? Maybe the solution exists for some other client+server pair?

  • Just curious, why are you trying to accomplish this? Is it in the case a computer is stolen to make it harder for the the thief to get access to the user's e-mail? – maik Dec 11 '09 at 17:56
  • Yes, pretty much it. I do not want to allow unauthorized access to the email data in case computers get stolen or confiscated (yep, such things happen in this part of the world). – kdl Dec 14 '09 at 11:53

I'm not sure how you're turning off Cached Exchange Mode in your testing, but Outlook 2003 and 2007 will function fine w/o creating an .OST file so long as Cached Exchange Mode is disabled.

Having said that, Cached Exchange Mode is helpful in many circumstances and, typically, doesn't impact the user negatively (since most PCs today have entirely too much local storage). Cached Exchange Mode can reduce server load by decreasing the I/O and CPU resources that users consume on the server (by servicing requests out of their local cache, rather than forcing the server to fulfill user requests).

Placing the Outlook cache on a "network drive" would partially defeat the purpose of the cache (which is to provide fast local access to remotely hosted data).

  • How I turned off cached mode: placed test PC into a separate OU, applied group policy from a template, and did some tweaking in Exchange mgmt console. Even though Outlook showed cached mode disabled (and I could feel some slowdown during certain operations), the OST file was created on a first connection to the server. Can you elaborate on how "Outlook 2007 will function fine w/o creating an .OST file" please? Is there some setting to tell Outlook not to create the file? – kdl Dec 12 '09 at 13:10

As Evan says, disabling OST files/cached mode is guaranteed to place more load on your server/s, it's not something you should do lightly.

Furthermore, placing PST/OST files on network shares is both unsupported and a bad idea.

Perhaps if you describe your reasons for wanting to do this, the community might have further advice.

  • Our mail traffic is quite low - a few hundred messages a day, so I guess our server will handle an increased load. As per PST files, I knew this is bad and they also get corrupted when placed on a network share and the connection gets broken. But I imagined OST files would do better since even if anything happens with it, it is recreated on a next connection to Exchange server. Guess I was wrong... – kdl Dec 12 '09 at 13:18

In my case I run Windows on a 10 Gb XP virtual machine, so I prefer not to add 3Gb of non-essential Outlook stuff to it. (just so the whole VM backs up more easily) While it's a network share from Windows point of view, it's actually a file on the very same harddisk, accessed through the virtual 'network' from the VM.


This can be a confusing topic but here's the scoop:

I'm guessing that a small ost file is being created, on the order of 265KB. This ost file is created as a result of the Offline Folder settings on the advanced tab of the Outlook profile properties. This is normal. This is not cached mode. These are Offline Folders. This is not a copy of the mailbox. The ost file you're seeing is for the Offline synchronization of mailbox folders configured for Offline synchronization in your Send\Recieve groups. If you don't want any ost file created then disable Offline Folders by selecting that option in the Offline Folders settings (again, on the Advanced tab of the Outlook profile properties. While Offline Folders and cached mode are very similar, they are not the same thing. Cached mode keeps a complete copy of your mailbox in the ost file while Offline folders keeps a copy of only those folders configured for offline synchronization.

  • Also, any ost or pst file that is 265KB is empty and is essentially a "placeholder" for the data that would normally be in a ost or pst file. a 265KB ost file has no folders configured for offline synchronization. A 265KB pst file would be created for any Outlook profile configured in POP mode even if that Outlook profile never connects to a mailbox. – joeqwerty May 29 '10 at 1:21

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