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Microsoft Outlook for Windows is configured by default in cached mode, whereby it stores a local copy of your mailbox on your computer.

If unauthorised access is gained to your computer it is relatively easy for this cached data to be exposed. This may not be desirable if you send or receive confidential or sensitive data and other precautions have not been taken to mitigate the risk, e.g. Whole Disc Encryption.

Unauthorised access to an Outlook cached mailbox is relatively easy to achieve and so Outlook should be configured to use non-cache mode where a heightened risk exists.

However, use of non-cached mode may result in some performance degradation.

IT Services publish instructions for changing Outlook to use non-cached mode in Microsoft Windows. The 'Configure Outlook 2010' utility provided by IMSU installers will configure an account in non-cache mode. When changing from cached to non-cached mode, you should delete the .ost file associated with the account.

Similarly, any other e-mail client that that stores a copy of e-mail locally is not recommend to be used where confidential or sensitive information is relayed. In this situation 'Outlook Web Access' (OWA) can be used & accessed via: https://owa.nexus.ox.ac.uk/

Outlook 2011 for OS X only supports cache mode, and so e-mail is retained locally.  Therefore Whole Disc Encryption may be appropriate if you send or receive confidential or sensitive data by e-mail via an Apple laptop. Alternatively only use the web access to your mailbox & don't download sensitive documents.