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Exchange Cached Mode

3 min. readlast update: 10.17.2023

What is Exchange Cached Mode in Outlook?

Cached mode, first implemented with Microsoft Outlook 2003, keeps a local copy of the user’s mailbox stored on the hard drive as an OST file. When Outlook is running in cached mode, the email client looks to the local OST file for all access, including reads and searches. At the same time, a separate process checks for new mail or other data from the server and syncs this to the local cache. 

When it comes to connecting an Exchange type E-Mail account to Outlook, it can be done in two modes: cached mode and online mode.

Exchange Online Mode basically takes all data requests directly from the Exchange server. This is ideal for users who do not have much disk space. However, this mode can be problematic as you can only access your data whilst an internet connection is available. If the connection drops, Outlook becomes unusable until the connection is restored. Latency-related issues should be also taken into consideration when choosing online mode.

Exchange Cached Mode gives you a better experience when you use an Exchange account. In this mode, a copy of your mailbox is saved on your computer to a .ost file. This copy provides quick access to your data, and it is frequently updated with the server that runs Microsoft Exchange. Of course, you will require much more disk space to store all data if you have a larger mailbox, but that's about the only drawback of having cached mode enabled.

Which mode should I use?

For all Office 365 users, the answer is simple - use Cached Mode. This will offer faster data access and data availability when in offline mode, with the only drawback being the use of additional disk space.

For corporate users, Online Mode is preferred for security reasons, as locally stored data is more vulnerable to data loss or malicious tampering. The security standards of the Exchange server offer far greater protection for sensitive information than users' typical antivirus and malware protection.

Why is this important for 4Team Products?

Most of our 4Team applications require an Exchange account in Outlook to be set in cached mode to communicate with the data inside. If it is set in online mode, our applications will be unable to access to your Exchange account and will not function properly. 

How to turn on Cached Mode in Outlook?

Step 1. Launch Outlook and click the File button in the top left corner.

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Step 2. In the Info section, click the Account Settings button and select Account Settings from the dropdown.

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Step 3. In the Account Settings window, select your Exchange email account and click the Change... button.

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Step 4. In the Exchange Account Settings window, make sure the check-box for Use Cached Exchange Mode... is enabled and click Next. You can also adjust the timeframe for old email downloads.

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Step 5. Click the Done button to successfully complete the update.

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If this is done for the first time, close Outlook via File > Exit, and re-launch the application once more. It will then create an offline .ost file and start filling it up with data. Depending on the size of your mailbox, this process may take a while.

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