Control Microsoft Outlook .PST file size and usage via the registry

Learn how to control and prevent Microsoft Outlook .PST problems by implementing registry keys that manage ANSI- and Unicode-based .PST file size and usage.

When users are close to hitting their mailbox quotas, they often avoiding deleting email by shuffling messages into Microsoft Outlook .PST files on local or network drives. This can cause unintended long-term consequences, including data loss and corruption, storage strain and .PST file format issues. Learn how to control and prevent Microsoft Outlook .PST problems by implementing registry keys that manage ANSI- and Unicode-based .PST file size and usage in Outlook 2003 and Outlook 2007.

Many use .PST files to evade Exchange Server's mailbox quotas. Instead of deleting messages when mailboxes begin filling up, users often move them to .PST files and keep them indefinitely. While this sounds like a solution, there are several problems associated with using .PST files this way.

First, storing email messages in a local .PST file long-term can result in unintended data loss. If a user stores his .PST file on a workstation's hard drive, the .PST file probably won't be backed up.

Second, Microsoft doesn't recommend storing actively used .PST files on a network drive, because corruption can occur if the network connection were to fail. Despite this, many organizations do, and the space that those files consume becomes an issue. Even if an Exchange server's hard drives are large enough to store the files, backup hardware may be inadequate to process such large files within the allotted backup window.

While these issues can be dealt with through proper planning, many administrators overlook a third issue that results from the existence of two different types of .PST files.

Prior to Outlook 2003, .PST files were ANSI-based, and had a strict 2 GB size limit. Originally, Microsoft Outlook didn't contain any method to prevent these files from growing beyond that size limit. Once an ANSI-based .PST file exceeds 2 GB, the file becomes corrupt.

Microsoft's PST2GB utility can be used to repair a corrupted Exchange Server .PST file. However, this utility truncates the file, which will result in some data loss.

Microsoft eventually implemented some controls in its Office 2000 Service Release 1A and Office XP SP1 to prevented .PST files from exceeding the 2-GB limit. The Office 2000 service release prevents .PST files from growing too large, while the Office XP service pack generates a warning when .PST files reach 1.8 GB. These controls are implemented in Outlook, not in the .PST file itself. This is important because any version of Microsoft Outlook can open ANSI-based .PST files.

Microsoft Outlook 2003 and Outlook 2007 use a Unicode format for .PST files. Unicode-based .PST files have a 2 to 20 GB size limit.

This 20 GB size limit presents some issues:

  • Space: If every user stored large .PST files on the Exchange server, you would encounter a storage issue.
  • Legacy ANSI-based files: Users can still open files from an ANSI-based .PST file in Outlook 2007, and then copy data to the file for either permanent storage or to relocate the data. Because ANSI-based files are subject to the 2 GB limit (even if opened in Outlook 2007) there is no way to upgrade the .PST file to Unicode format. You can create an empty Unicode-based .PST file, and move the data from a Unicode .PST file into it, but you cannot upgrade.

To prevent problems with mismatched file limits and excessive storage space requirements, some Exchange Server administrators implement registry settings that control the behavior of both ANSI- and Unicode-based files.

Keep in mind: Editing the registry can be dangerous. Making an incorrect modification can destroy Windows and, possibly, your applications. I recommend implementing a full-system backup before editing the registry.

The location in which you must create registry keys varies, depending on whether you're using Outlook 2003 or Outlook 2007. In Outlook 2003, the registry keys must be created at:


In Outlook 2007, registry keys must be created at:


For several registry keys, Unicode key values must be entered in terms of megabytes, but expressed in hexadecimal notation (Table 1). For example, 20 GB is the same as 20,480 MB; this would be entered as 5000 in hexadecimal notation. Values for ANSI-related keys must be entered in terms of bytes, but expressed in hexadecimal notation.


Value Range 

Default Value 



0x00000001 (1 MB) to 0x00005000 (20 GB)

0x00005000 (20 GB)

Maximum size of a Unicode .PST file.


0x00000001 (1 MB) to 0x00005000 (20 GB)

0x00004C00 (19 GB)

The threshold at which a warning is displayed when Unicode .PST files become too large.


0x001F4400 to 0x7C004400

(1.95 MB to 1.937 GB)


(1.933 GB)

Maximum size for an ANSI based .PST file (the maximum size is set to just under the 2 GB threshold).


0x001F4400 to 0x7C004400

(1.95 MB to 1.937 GB)


(1.816 GB)

The threshold at which a warning is displayed when Unicode files grow too large.

Table 1. Registry keys that control .PST file sizes.

NOTE: If you need help converting decimals to hexadecimals, the calculator found in Microsoft Office Windows can do this, when operating in scientific mode.

About the author:
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). 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 Web site at www.brienposey.com.

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