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What is "Cached Exchange Mode" and
When Should You Use it?

When you're setting up Outlook to connect to an Exchange Server in "native" mode, there's an option to Use Cached Exchange Mode which you can choose to select or deselect:-

Outlook profile setup page where you select Cached Mode

An Explanation of Cached Exchange Mode

In Cached Exchange Mode, a full copy of all your email folders, contacts, calendar items etc. is kept on your PC's hard drive in a file, usually called outlook.ost and located in:-


C:\Documents and Settings\<your username>\Local Settings\Application Data\Microsoft Outlook\

The contents of this OST file is kept synchronised with the master copy on the Exchange server.
An OST file is similar to a PST file which stores all your Personal Folders data when Outlook is used in standalone mode instead of in conjunction with an Exchange server.

The advantages of using Cached Exchange Mode are:-

1 -

If the connection between Outlook and Exchange is lost, you can carry on working: reading and creating emails and editing other information.
This is called Offline Mode and in this mode emails can't be sent or received until the connection with Exchange is restored.

2 -

If the connection to Exchange is lost permanently due to some disaster or dispute, you can copy all your Exchange data from the OST file to a locally stored PST file, obtain a POP3/SMTP email account and continue to operate independently with no loss of data.
While most Exchange servers have a good backup system, your OST file is your personal backup which is under your control.

3 -

If you accidentally delete everything in your Inbox, bypassing Deleted Item, and the retention period on the server expires or you can't find anyone to restore your Inbox from a backup, your OST file can come to your rescue.
Not the OST file on the computer you performed the deletion on but, if you use Outlook in Cached Exchange Mode on another PC, perhaps your home PC, you can turn this PC on (after first disconnecting it from the Internet so that it doesn't synchronise with your, now empty, Inbox), copy the contents of your Inbox to a new Personal Folders Inbox, reconnect to the Internet and finally copy everything back to your Exchange Inbox from the Personal Folders Inbox.
After synchronization, everywhere you use Outlook will now have your Inbox restored.


The disadvantages of using Cached Exchange Mode are:-

1 -

Cached Exchange Mode operates more slowly that Non-Cached Mode.
This is most noticeable when your PC is on the same local network as Exchange and so has a very fast connection to it.
You won't notice much difference connecting to Exchange over the Internet.

2 -

If you have a large amount of data in your Exchange mailbox - perhaps 500mb - it will take a very long time to perform an initial synchronization over the Internet and the 500mb OST file will occupy a substantial amount of space on your hard drive and cause Outlook to take longer to start.

3 -

There will be a copy of all your Exchange data on every PC on which you use Outlook in Cached Exchange Mode.
Although the OST file format has built-in security to stop it being read easily, there are now several utilities than can convert an OST file to a PST file and so, using
Cached Exchange Mode means your Outlook Data is less secure against being accessed by others.

Our Advice on When to Use Cached Exchange Mode

Use Cached Exchange Mode on at least 2 secure PCs, especially in situations where a permanent connection to Exchange is not guaranteed.

Don't use Cached Exchange Mode if you're setting up Outlook on a PC on a temporary basis, say, on someone else's PC.
In such circumstances consider making do with OWA

To improve Cached Exchange Mode performance, keep your Outlook data to a minimum by deleting unimportant emails, archiving important ones over a year old, and regularly emptying your Junk E-Mail and Deleted Items folders.
I've seen OST files grow to over 3 gigabytes.

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