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Outlook 2003 Cached Exchange Mode

By mm212 ·
Many people in my organization like to use Cached Exchange Mode in Outlook 2003. One thing we are seeing is confusing me. The way I understand Cached Exchange Mode to work, Outlook leaves all the messages on the Exchange server, but downloads a copy of all the messages onto the local workstation, saving them in an .ost file.

What we're seeing on just a couple of accounts, not all, is that if you get the folder size information through Outlook 2003, it might show a mailbox that is 48MB in size (taking one account as an example). If I look in Exchange System Manager, this person's mailbox is 2MB in size. Needless to say, this user doesn't want to lose their mailbox if their computer crashes or if I switch them so they are not using Cached Exchange Mode. Can someone tell me what is happening here? Does the server still hold all their messages? If so, why doesn't the mailbox size come close to matching?

Thanks for your help!

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by GunnerSixz In reply to Outlook 2003 Cached Excha ...

You might want to make sure that some of the users have not also defined send and receive groups to the OST. With Exchange cached mode you do not need to define these folders. In Outlook 2000 you had to define these groups. I'm thinking that the reason for the incosistency is that the option is enabled and it is only caching some folders. Also, check to see if the security settings are so that the outlook will not download attachments and leave them on the information store until you click on them on demand. I hope that helps.

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by mm212 In reply to

I checked what you recommended. There are no send and receive groups defined. Regarding the setting to not download attachments and leave them in the message store until they are opened, this setting is still the default. I also don't think this is the issue since the mailbox size that shows up in Outlook is much larger than what is in the message store. I appreciate the answer, but this didn't fix it. Thanks!

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by brasslet In reply to Outlook 2003 Cached Excha ...

Cached mode does indeed leave the messages on the Exchange server. If you or your user are worried that this is not the case, you can prove it by having the user log on to the mailbox via OWA or from another workstation. The mail will all be there, as long as the user is simply using the cached mode and not taking mail off the server via other means (POP3, delivery to a .pst file etc.).

What you're likely seeing is that the locally cached mail does not take advantage of single instance storage (SIS). SIS means that attachments sent to multiple users on the same server are stored only once on the server, everyone gets a pointer to this attachment. This feature helps keep the size of the information store down. Mail that is taken off the server, whether in Outlook cached mode, a .pst file, POP3 etc. will no longer use SIS as it is no longer on the server. A 2Mb to 48Mb difference is very large, but it is entirely possible depending on how your organization's users utilize Exchange.

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by mm212 In reply to

This doesn't seem to be the case. When looking at even the individual message size, every message shows a larger size when in cached mode than in Exchange mode. This happens even on messages that were sent to only one person. It almost seems as if it's some sort of compression going on. I can't make sense of it, but neither can any other other Exchange admin I know. They never noticed it before, but they see the same thing. It doesn't seem to be causing any problems so I'll just chalk it up to a mystery until I stumble across the answer sometime in the future.

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by mm212 In reply to Outlook 2003 Cached Excha ...

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