Configurations that can Compromise the Effectiveness of Outlooks Cached Mode

Of all the features included in Microsoft Outlook, my favorite has to be cached mode.  In case you’re not familiar with cached mode, it is a mechanism that copies the contents of a user’s mailbox, calendar, etc. into an offline file called an “OST” file.  This allows the user to have full access to anything in the cache, even when the user is not connected to the Exchange Server.

Why can Outlook Offline Cache Mode cause problems?

The reason why I like cache mode so much is because I tend to get a huge volume of e-mail.  Whenever I travel, I can spend my time on the flight reading and responding to my e-mail.  Granted, only the messages that I have received as of the time that I disconnected from my Exchange Server are cached, and none of my responses will actually be transmitted until I can connect to my Exchange Server again. However, being able to read and compose messages while off-line is a huge timesaver for me – and I know it would be for you too.

Another reason why I like using cached mode is because it provides a degree of fault tolerance.  Remember, the entire contents of a user’s mailbox are copied to an OST file that resides on the user’s local hard drive.  I once had a client who’s Exchange Server failed catastrophically.  As it turns out, the client had never made a backup of their exchange data.  To make a long story short, I was able to use the cached messages from each user’s OST file to rebuild the information store.  Obviously, this is far from being an ideal disaster recovery solution.  In that particular case though, having the OST files meant the difference between recovering or not recovering data.

If you are unfamiliar with where to configure Outlook Cache mode, here is what it looks like:

outlook cached mode configuration

What are the problems with using Outlook Offline Cache Mode?

As great is cached mode is though, there are some situations in which you may want to think twice about using it.  Generally speaking, cache mode works fine in almost every situation as long as the end user has connectivity to Exchange.  It’s when the user goes off-line that things can get a little weird.

Delegate Access Issues

One example of such a situation would be a case in which a user performs delegate access to another user’s mailbox or accesses a shared folders or calendars.  The reason why this tends to be a problem is because cache mode does not download another user’s data to the OST file.  If a user is used to being able to access another user’s mailbox or a shared folder or shared calendar, they may be in for a bit of a letdown if they ever have to work off-line.  In such a situation, Outlook will initially appear to freeze, but will eventually timeout and allow the user to access their cached data.  Of course the cache will only contain the user’s own data, not any shared data that the user may normally work with.

Public Folder Issues

Public folders can cause similar types of problems.  When a user is working off-line, they will not have access to public folders by default.  It is possible to use Outlook’s public folder favorites feature to synchronize frequently used public folders to the off-line cache.  You have to be careful using this feature though, or you could accidentally end up synchronizing an excessive amount of public folder data.

Custom Address Book or Contact List Issues

Another situation that can cause problems with cached mode is the use of custom address books and contact lists.  If the Exchange organization includes custom contact lists or address books that are designed to be searchable, then communications with the Exchange Server are required in order to perform a search.  That’s because these types of address lists and contact lists are not cached by Outlook.

Digital Signature Issues

One last issue that I want to talk about is the use of digital signatures.  If a message is sent using digital encryption, then Outlook must check to make sure that the message’s digital signature is valid.  This almost always requires a call to the server.  Because of this, users may not be able to access encrypted or signed messages while working off-line.


As you can see, Outlook’s cached mode can provide users with a great deal of flexibility if they need to work off-line.  However, certain Exchange features don’t work very well with cached mode, and may cause unexpected results when a user is working off-line. Keep these in mind when using Outlook / Exchange OST Cache Mode.