After Microsoft released its February 2009 cumulative update for Outlook, you can have a larger mailbox than was...
previously possible. Before Outlook was updated to improve performance, it was recommended that Exchange 2007 mailboxes not exceed 2 GB in size.
Although it's possible to have a larger mailbox, once it exceeds 2 GB Outlook's performance begins to diminish if there isn't sufficient hardware on your workstations. After applying either the cumulative update or Office 2007 SP2, you should keep user's mailboxes under 5 GB. Mailboxes larger than 5 GB can diminish Outlook performance.
Applying Office 2007 SP2 and keeping mailboxes under 5 GB are just two ways to improve Outlook performance. How data is stored within a mailbox also has a lot do with how Outlook performs.
Suppose that you have a 3 GB mailbox and that you're running Outlook 2007 in cached Exchange mode. In this scenario, you also have a high-end PC running Office 2007 SP2. You're well within the general recommendations, so Outlook should perform flawlessly, right? Don't bet on it. Even though this technically conforms to the recommended best practices, performance can still suffer, especially if all 3 GB are comprised of messaging data that's stored in the inbox.
In my experience, there's a threshold in which some messages become inaccessible if exceeded. In our 3 GB inbox example, a few thousand of the more recent messages would be accessible, but Outlook wouldn't acknowledge the existence of older messages.
This doesn't mean that the older messages have been deleted or are permanently inaccessible. To retrieve them, take Outlook out of cached Exchange mode. The downside is that when you do, Outlook performance may begin to suffer.
The question of how to ensure Outlook performance and message availability, even when users have large mailboxes, remains. The answer is simple: Don't place all messages into a single folder.
Some Microsoft knowledgebase articles imply that you have up to 10,000 items in a single folder (50,000 items with SP2), but I've often experienced problems with fewer items. If users retain messages over long periods of time, you may want them to create folders that organize messages according to year. That way, they can avoid having too many items in one folder.
If you're running Exchange Server 2007, use managed folders to maintain the health of user mailboxes. Managed folders are used to specify retention rules. For example, you could use managed folders to ensure that users don't keep items in their inboxes for longer than specified.
About the author: Brien M. Posey, MCSE, is a five-time recipient of Microsoft's Most Valuable Professional (MVP) award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), and File Systems and Storage. Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.
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