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Predict how large Outlook 2007 mailboxes will perform

Despite Outlook improvements in Microsoft Office 2007 SP2, mailboxes that are larger than 5 GB can still experience performance problems. Use these recommendations and the WinSAT tool to accurately predict how large Outlook mailboxes will perform.

With the release of Office 2007 SP2, Microsoft changed how .ost files function and its .ost file usage recommendations. Previously, Microsoft recommended that a user with a 1 GB mailbox operating in cached Exchange mode should have at least a 5400 RPM hard drive and 1 GB of RAM. For mailboxes up to 2 GB, Microsoft recommended that workstations have at least a 7200 RPM hard drive and 1 to 2 GB of RAM if operating in cached Exchange mode.

These recommendations change for mailboxes larger than 2 GB. Some resources state that you should either reduce the mailbox's size or avoid using cached Exchange mode. Other resources state that if you have sufficient hardware, you won't see any performance problems until your mailbox size exceeds 4 GB.

More on Microsoft Outlook:
Avoid Outlook 2007 performance issues during repairs

Understanding Microsoft Outlook 2007 data file usage

Control Outlook 2007 in cached mode settings with group policies

Actually, I've found that you don't have to worry about mailboxes 5 GB in size or less. Outlook shouldn't experience any performance problems with those size mailboxes on most current hardware.

However, when the size of a mailbox grows greater than 5 GB, things become less predictable. Mailboxes between 5 and 10 GB in size may cause Outlook to occasionally pause, especially on computers with slower hard drives.

Performance problems begin to occur on most current hardware when mailbox size exceeds 10 GB. Performance further breaks down when mailboxes grow to about 25 GB. At this point, Outlook performance will become choppy and pauses will become more frequent.

When a mailbox grows this large, consider taking action to improve Outlook performance by taking the following measures:

  • Archive older messages to reduce mailbox size.
  • Use online mode instead of cached Exchange mode.
  • Use send/receive groups to manually synchronize a mailbox.

Most of the time, you can use these methods to predict how Outlook will behave on a specific hard drive and mailbox. However, to get a better idea of what kind of performance to expect, Microsoft recommends using its WinSAT tool to measure a computer's noncontiguous write performance. The recorded value can give you an idea of what to expect from Outlook's performance.

The WinSat tool is included with the Windows Vista Upgrade Advisor 1.0. To get the necessary benchmarks, enter the following command:

WinSAT –Drive c –Ran –Write –Count 10

You can then find the results in the C:\Windows\Performance\Winsat\Winsat.log file.

The following table illustrates typical Outlook performance results.


Outlook 2007 SP1 (without the February 2009 update)

Outlook 2007 SP2

15 Mbps or less

Not supported

Reasonably responsive, but with occasional pauses

15 to 30 Mbps

Frequent pauses

Reasonably responsive, but with occasional pauses

30 to 40 Mbps

Reasonably responsive, but with occasional pauses

Very responsive, but with occasional pauses

40+ Mbps

Very responsive, but with occasional pauses

Very responsive and pauses are rare

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

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