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Tuesday, September 05, 2006

Outlook: Demystifying Outlook Cached Mode


Outlook 2003 provides a neat feature called Cached Mode. When running Outlook 2003 in Cached Mode, you have the option to download all messages to a local .OST file (similar to a .PST). Thus most of your Outlook tasks is performed locally rather than from your MB that's stored on the server side. This reduces server load and network bandwidth. The following provides list provides an overall picture of running Outlook in Cached Mode.

1. Reduce server load and network bandwidth since tasks/operations are performed locally.

2. You have the option to download all messages when starting Cached Mode or to download only headers such as in IMAP

3. If network connectivity is lost, you still have access to existing content.

4. Some features are still performed on the server side such as Out of Office messages and processing of Outlook rules

5. A common issue user's experience is not getting a real time Global Address List. For example, if a new user is created, a user in Cached Mode may not see this new user in the Global Address Book. This is because when in Cached Mode, you download a copy of the GAL once a day which is by default. You can force to re-download the GAL through Outlook Send\Receive settings. Another option is to use a registry hack

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Outlook\Cached Mode

Value name: ANR Include Online GAL
Value type: REG_DWORD
Value: 1 (1=enabled, 0=disabled)

Additional Resources with GAL and OAB

How to configure how the Offline Address Book is downloaded when you use Outlook 2003 in Cached Exchange Mode

Administering the Offline Address Book in Outlook 2003

6. Outlook Cached Mode works best with MB's less than 1GB in size.

James Chong
MCSE M+, S+, MCTS, Security+

How useful was this article? Want to see a tip not listed? Please leave a comment.


Anonymous Jorge Gallardo said...

Very interesting.
I'm going to check it in my organization.

Thank for share your knowlegde.


10:02 AM  
Blogger jamestechman said...

Hello Jorje, glad you found this helpful. As always if you have any questions, you can email me.

7:46 PM  
Anonymous Joel said...

Why best under 1GB?

11:30 PM  
Blogger jamestechman said...

Hello Joel. There is no magic figure to say where cached sized mailbox performs best. However with any large file, .pst, .ost when you have many items, the performance starts to degrade when you perform functions such as performing categorized views. I've also seen some weird Outlook issues when users started exceeding 1-2GB range. Whether or not this is directly related to the size of the .ost file, something else or a combination of both, it's always best to keep your cached MB as low as possible.

For more info please refer to

"One factor that can contribute to reduced performance is a large OST file. If the user's OST file grows too large (for example, larger than 1 GB), Outlook with Cached Exchange Mode performance degrades."

2:34 PM  
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Blogger Alexis said...

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6:27 AM  
Blogger Alex said...

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9:44 AM  
Blogger Alexis said...

I work with mails very often and consequently I had many problems,and me helped-.pst view.As far as I know tool is free,and besides that software can convert your file to a file with *.pst extension, that follows the original structure.

10:18 AM  
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9:04 AM  
Anonymous Anonymous said...

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11:54 PM  
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9:46 AM  
Anonymous Jim Rene said...

For fixing corruption and other related issues in MS Outlook, here are some tips and methds which can help you to solve your problems and and also various questions like how to convert ost to pst file: Tips to Fix OST file and how to Convert OST to PST file


1:52 AM  
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1:36 AM  
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