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white paper
The PST
File Rodeo
By Paul Robichaux
Outlook personal folder files, better known simply as
PST files, have been around for a very long time: they
were first introduced with the Exchange client that
shipped with Exchange 4.0. PST files allow users to
manage their own mail storage and archiving, and for
that reason they are widely used within organizations
of all sizes. Users love having a convenient, portable,
personal message store. On the other hand, companies
want to be able to manage, monitor, and control infor-
mation on their networks, especially as compliance and
electronic discovery have grown in importance since
the time when Exchange 4.0 shipped. Striking the right
balance between these two opposing sets of demands
is tricky, although Microsoft has tried. This paper will
discuss the broad history and functionality of PST files,
point out some specific dangers and benefits of their
use, and describe methods you can use to control (or
prevent) the use of PST files in your organization.
white paper The PST File Rodeo
contents
Understanding
PST Files
page 2
A Brief History of
PST Files
page 3 Understanding PST Files
One of the key concepts email users have had to absorb is that
PST vs. OST Files their mail can be stored on a central server, not on a local com-
page 3 puter. In the days when for-pay services like CompuServe and AOL
dominated the email landscape, and company-provided email
PSTs: was relatively rare, it was natural to think of email as something
Pros and Cons you downloaded to your computer and stored locally. Most us-
page 3 ers work with email on a single computer, and as a result many
people still have the idea that email is stored and managed locally.
PSTs and (Ask around your office or dinner table if you do not believe that
Exchange 2010’s this is true!)
Archiving
page 4 PST files were originally introduced to give users a means of locally
storing messages that could tie in with Exchange Server-based
Managing PSTs storage. As originally envisioned, PST files were meant to:
page 5 • allow users to retain access to their email even while disconnected
(an especially important feature given that dial-up access was the
norm when this feature was designed!)
• provide a simple way to move mail content from place to place
• give users a way to manage their own message storage, freeing
up resources on the server
Don’t be too hard on Microsoft for the faults of PST files. The
constraints that Microsoft had to meet with the initial design for
PST files were pretty daunting—an email server might have a few
hundred megabytes of space shared among all mailboxes on the
server, while individual clients might have more. Multi-gigabyte
mailboxes, or mailboxes with hundreds of thousands of items,
were pretty much beyond imagining at the time, which is why the
original PST file format couldn’t handle them.
sponsored by The PST File Rodeo 2
white paper The PST File Rodeo
A Brief History of PST Files PST vs. OST Files
The objects we think of as “PST files” have undergone a number of One point of confusion that still lingers is the difference between
major changes since their original introduction. The Outlook PST PST files and the similar-sounding offline folder file (OST) format.
format we know and love was first introduced with Outlook 97, While the PST format is designed to serve as a more or less static
where it was officially known as the “Microsoft Outlook Personal container for mail, the OST file is best thought of as a cache that
Folders file.” It allowed users to store up to 2GB of mail data in holds a copy of all the data in a user’s mailbox. When Outlook is
a single file, with a limit of 65,000 items per folder. This format running in cached Exchange mode, updates from the server are
suffered from a number of limitations, though. First was the 2GB applied to the OST file, as are changes the user makes locally.
limit; that seemed like a very high limit back in 1997, but as disk
capacities steadily grew, users started to run into the limit. A When Outlook is not connected to the Exchange server, the user’s
bigger problem was that the PST structure itself was brittle and changes are still applied to the OST file; when Outlook reconnects,
easily corrupted. Transient problems with disk hardware, flaws in any changes made to the Exchange mailbox or the OST file are
Outlook and Windows, or plain old random bad luck could quickly synchronized automatically.
render an entire PST file unreadable—but so could adding data
that pushed the file past its 2GB limit! These problems were com- Contrast this with a PST, which does not have a server-side
pounded by the fact that most people did not bother to back up counterpart. If your Exchange profile is set to deliver mail to a PST,
their local PST files, so once a PST file was damaged its contents when Outlook connects to the server, the mail is automatically
might be unrecoverable. In addition, the original PST file format downloaded and stored in the PST file, then removed from the
couldn’t handle content using the Unicode system of content server. Whatever changes you make to the local PST file, remain
encoding. in the file; they are never replicated back to the server. Depend-
ing on why you are using PST files, this may or may not be a good
To address these problems, Microsoft introduced a new file format thing.
with Outlook 2003. To help keep things straight, the new format
got a new name: the Microsoft Office Outlook Personal Folders There is an additional wrinkle, too. Each OST file is linked to a spe-
file1. This new version removed the 2GB size limitation, as well as cific Outlook profile on an individual computer by the presence
the 65,000-item cap, and it supported the Unicode character en- of an encrypted token. If you delete the profile, or move the OST
coding system. By default, Outlook 2003/2007/2010 Unicode PST to another computer, you can no longer open it. By contrast, PST
files can grow up to 20GB, although the actual limit can be config- files are designed to be freely moved between machines.
ured by setting a registry key. As part of the new format, Microsoft There are a number of commercial tools that can convert OST files
made internal format changes to help make PST files more robust. into PST files. However, the simplest way to do this is to open the
These changes are invisible to users, except that some errors that OST file, create a new PST, and manually move content to it. That
would have rendered old-format PSTs unreadable are now fixable. does, however, require you to be able to open the OST file; so if
you really need to recover content from an OST file, you may end
Outlook 2010 did not make any changes in the PST file format (at up using a commercial tool.
least not that Microsoft is admitting). Some of the much-heralded
performance improvements in Outlook 2007 SP2 were actually
fixes from Outlook 2010 that were backported to the earlier ver- PSTs: Pros and Cons
sion, but there do not seem to be any major changes to the PST Because PST files have been around for so long, many Exchange
file format itself. administrators think of them as a natural part of the Exchange
landscape. Like other flora and fauna though, some PSTs are
noxious, unwanted, and even dangerous (like, say, black widow
1 If you didn’t notice much difference between the two names, you’re not alone. Most spiders or rattlesnakes), while others see them as useful, like
Outlook experts call the Outlook 2003-and-later versions “Unicode PSTs” and the older versions
“ANSI PSTs.” mosquito-eating bats. A thorough examination of the pros and
sponsored by The PST File Rodeo 3
white paper The PST File Rodeo
cons of PST files will help you better understand how they might have any redeeming features. (In fact, for a good laugh, plug “PST
fit into your landscape. advantages” into your favorite search engine and examine the
results—none of them actually talk about PST advantages!) The
The Dangers of PST Files truth is that the biggest benefit of PST files is their familiarity to
One of the first things I learned as a neophyte Exchange adminis- users; once users become accustomed to storing their own mail
trator was a simple rule: PST = BAD. There are ample justifications data, using their own preferred filing system that is totally under
for that rule, including these: their control, it can be difficult to win them back. PST files do offer
some other advantages though:
• Because PSTs are stored on individual workstations, they frequent- • They are intentionally designed to be portable. You can put a PST
ly do not get backed up. That means critical mail data may exist file on a thumb drive or another storage device and open it from
in only one place. As an additional insult, users who move their any machine that has Outlook installed, which is often easier than
PSTs from place to place (or who keep multiple copies) can become creating a new Outlook profile or otherwise connecting to your
confused at the lack of integration between their PST file(s) and actual Exchange server.
their online mailbox. • They allow users to store mail in a way that does not count
• Neither Windows nor Exchange provides any tools to inventory or against the user’s storage quota. This can be useful for users who
track the contents of PST files; so there is no built-in way to find out have a genuine business need to keep more mail around than will
what PST files exist on your network or what is in them. fit under their actual quota.
• Because users control the contents of PSTs, they may freely violate • The PST file format is a standardized way to import and export
your organization’s message retention and management poli- mail content. Microsoft’s own tools (including the Exchange
cies—and they can probably do so with impunity, given the lack of 2007/2010 import-mailbox and export-mailbox cmdlets and
built-in PST management tools. the recovery storage group tools included in Exchange 2003) can
• PSTs are normally tied to a single computer, and they cannot be produce and consume PST files, as can many third-party archiving
viewed within Outlook Web Access or on mobile devices. Users who and management tools.
routinely switch between OWA, mobile devices, and Outlook will • They give users a location to store personal or sensitive content
only have partial access to their mail. that they do not want read by nosy administrators.
• Microsoft has improved the robustness of PST files since their • They provide a storage mechanism for user mail that Outlook
introduction, but they are still relatively easy to damage or corrupt retrieves from external IMAP or POP accounts.
(compared to, say, a Word document or a TIFF image). • They provide an inexpensive, albeit limited, way to archive mail.
• PST files can be password protected, but the algorithm used is Although it is certainly not advisable to rely on PSTs as a full
relatively weak. This can sometimes mislead users into believing archiving solution.
that their data is more secure than it really is.
• PST files are difficult, even impossible, to use on other platforms. PSTs and Exchange 2010’s Archiving
There are few reliable tools for opening and working with PST files Speaking of archiving, it is worth mentioning the new archiving
on Mac OS X or Linux computers. features in Exchange 2010. They are designed to provide reten-
• Versions of Outlook prior to Outlook 2007 SP2 are slow to read tion and control features (including litigation holds and a much-
and write PST files, particularly if the PST files are stored on network improved cross-mailbox search experience) that were not part
shares. In fact, Microsoft’s official stance is that they do not support of Exchange 2007, as well as the ability for users to create and
the use of PST files stored on network shares because doing so can manage their own personal archives without using PST files.
cause the file server to exhaust its non-paged pool resources. The archiving functions in Exchange 2010 are well-described in
the product documentation, but there are a few highlights you
The Benefits of Using PST Files should know about:
After that long list of disadvantages you may wonder if PST files • The archive mailbox for a user has to remain in the same mailbox
sponsored by The PST File Rodeo 4
white paper The PST File Rodeo
database as the primary mailbox. This means that you cannot 4. Click the Start button. The Inbox Repair Tool will start its work.
separate archive mailboxes into their own databases for disaster When it is done, you will see a summary dialog like the one shown
recovery, backup, or high availability reasons. in Figure 1. You can click the Details button to see a summary of er-
• The archive mailbox is only available in Outlook 2010 and OWA rors found during the scan, although normally the summary will be
2010. There is no current way to access it from other clients. something meaningless like “Internal errors were found in this file.”
• The archive mailbox is only available when the user is online;
there is no way to work with an offline copy.
• Users can drag and drop items between their primary mailbox,
their archive mailbox, and individual PST files, but there is no exist-
ing way to import PST files directly to users’ archive mailboxes.
Managing PSTs
The bad news: you may not be able to eliminate PST files entirely.
The good news: there are some things you can do to help control
them. In addition, there are a few management tasks (like fixing
corrupted PST files) that you will probably need to know how to
do until you can eliminate them altogether.
Repairing PST Files
Microsoft has made available a repair tool that can sometimes
rescue the contents of a corrupt PST. According to its documenta-
tion, the Inbox Repair Tool (better known as “scanpst.exe”) does
four things:
• First, the Inbox Repair Tool makes a backup copy of whatever file
you provide it. Figure 1: After running ScanPST you can repair the file and optionally
• The Inbox Repair Tool will try to turn any file you give it into a PST see a list of errors
file. For example, if you rename an executable file to junk.pst, the
tool changes the file to a mountable .pst file. The results of this 5. Click the Repair button.
conversion may range from laughable to useful to dangerous. 6. When ScanPST has finished its repair, it will display a confirma-
• The Inbox Repair Tool analyzes the PST’s directory structure and tion dialog telling you so. Click OK.
item headers to try to recover all folders and items. 7. Launch Outlook and create a new PST file. This is where you will
• If the Inbox Repair Tool reports that it recovered the PST file, it copy information recovered from the repaired PST file.
means that the repair tool found problems, and repaired what it 8. Open the repaired PST file. You will notice that it contains a folder
could. In this case, you may still have lost data. named “Lost and Found.” This is where all your recovered items now
live. Be aware that depending on how successful the Inbox Repair
Running the tool is simple. Here is what you need to do: Tool was, this folder may not contain all of the items that you had
1. Quit Outlook. hoped to recover.
2. Launch the ScanPST.Exe tool. Depending on which version of 3. 9. Drag the recovered items from the Lost and Found folder to your
Windows you are running, it can be located in a number of differ- new PST file.
ent locations, so your best bet is to search for it.
3. Click the Browse button, then navigate to the PST file you want If there are particular items that you were hoping to recover that
to repair. the tool did not catch, you can open the backup file that the
sponsored by The PST File Rodeo 5
white paper The PST File Rodeo
Inbox Repair Tool creates; it will have the same path and name as application data folder, the name of which varies between differ-
the original file, but with a “.bak” extension. Copy the file, give the ent versions of Windows. However, you can force Outlook to place
copy a “.pst” extension, and then use Outlook’s Import and Export PSTs at a path you specify.
Wizard to try to import it.
The first route is to bundle the desired setting as part of the Of-
Setting a Maximum PST File Size fice installation itself. For Office 2003, you would use the Custom
You can restrict the maximum size that a PST can grow to, by add- Installation Wizard (CIW); for Office 2007, you will need to use the
ing registry keys to the appropriate version-specific path (HKEY_ Office Customization Tool (OCT). In either case, you can specify
CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\ the path where PST files will be stored. This will have no effect on
PST for Outlook 2007 and HKEY_CURRENT_USER\Software\Poli- existing PST files, but users will only be able to create new PST files
cies\Microsoft\Office\11.0\Outlook\PST for Outlook 2003). There at the location you specify. Unfortunately, this approach works
are four separate keys, as shown in the table below: best when you use it before installing Office, because the configu-
ration you specify is applied as part of the installation.
Name Applies to Valid range Default
The second method allows you to enforce a specific path at any
MaxLargeFile- Unicode PSTs Up to 20GB 20 GB
time, and as a bonus it can be applied to individual users. In fact,
Size (0x00005000)
it must be applied to individual users, so keep that in mind if you
WarnLarge- Unicode PSTs Up to 20GB 19GB
want to apply a restriction globally. To use this method, you will
FileSize (0x00005000)
need to add a new string value to the HKEY_CURRENT_USER\Soft-
MaxFileSize ANSI PSTs 0x001F4400 – 1.933 GB ware\Policies\Microsoft\Office\12.0\Outlook (for Outlook 2007) or
0x7C004400 HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\
(2 GB) Outlook (for Outlook 2003) keys. The new value should be named
WarnFileSize ANSI PSTs 0x00042400 – 1.816 GB ForcePSTPath, and it should contain the path you want to require
0x7C004400 users to use. Remember that when you put a string value in the
(2GB) registry that you must use “\\” to encode a single backslash, so the
correct value for a desired path of z:\email would be “z:\\email.”
As you can see, there are separate keys for ANSI and Unicode PSTs, Preventing the Use of PST Files
with different size limits for each. The Unicode format can theo- You can actually prevent Outlook users from accessing PSTs in
retically handle up to 33TB, but the maximum size you can specify several different ways.
is 20GB for Unicode PSTs and 2GB for ANSI PSTs. First, you can prevent users from creating PST files at all with the
DisablePST registry key (HKEY_CURRENT_USER\Software\Policies\
Interestingly, the behavior of these keys may be different from Microsoft\Office\12.0\Outlook\DisablePST), a DWORD value that
what you expect. The WarnLargeFileSize and WarnFileSize keys forces Outlook to remove the File | New | Outlook Data File and
do not actually generate a warning. Instead, they prevent adding File | Archive commands from the Outlook menu bar. This keeps
items to the PST file once it reaches the specified limit. However, users from making new PSTs, as well as blocking their ability to
Outlook itself can cause the file to grow by reindexing or changing import and export items with existing PST files.
items—so the PST file’s physical size on disk can still exceed the
limit. When you use the MaxFileSize and MaxLargeFileSize keys, You can also remove users’ ability to use Outlook’s AutoArchive
they set a hard limit on the file size, which Outlook will not exceed. features. Even though you block PST files, users can still use
AutoArchive unless you proactively disable it. To do so, you will
Controlling Where PST Files Go need to ensure that the ArchiveDelete, ArchiveMount, ArchiveOld,
By default, Outlook stores users’ PST files inside their profile’s local DeleteExpired, DoAging, and PromptForAging registry values (all
sponsored by The PST File Rodeo 6
white paper The PST File Rodeo
Managing PST Files
Access to Outlook Personal Folders (PST files) is necessary for legal discovery, regulatory compliance, and implementation of corporate policy. The
decentralized nature of PST files combined with their proliferation throughout the enterprise on desktops, file servers, and laptop machines often
make the task of finding, accessing and managing these information stores extremely difficult for administrators. To alleviate the headaches often
caused by PST files, Sherpa Software offers a variety of PST management tools specifically designed to locate, search, manage, migrate, and delete
content in desktop and network-based PST files.
Mail Attender Archive Attender Discovery Attender PST Backup
Mailbox, PST File and Public Mailbox, PST File and Public Electronic Search and Discovery Attender
Folder Management Folder Archiving Learn More PST File Backup & Synchronization
Learn More Learn More Learn More
Software Highlights:
Locate, Manage, and Search PST Files—PST content is not easily accessible. When data is stored locally on desktop or laptop computers, or
scattered around your network, finding PST files and collecting information can be a difficult and time-consuming task. In addition, enforcing archiving, re-
tention and deletion policies are much more difficult when data stores are not centrally accessible to IT staff. Sherpa’s products provide content discovery,
migration and PST management for local and network PST files by allowing administrators to create flexible rules that monitor the email infrastructure and
automatically archive and enforce corporate policies.
Archive and Delete PST File Content—Transferring PST content into an archive, between e-mail stores and giving users of those PST files
access to the archived messages through Outlook, Sherpa’s solutions allow you to delete unwanted PST files. Since messages from the PST files are now
available to be searched and restored from the archives, the use of PST files and the creation of new PST files are no longer necessary.
Backup and Synchronize PST Files—The practice of storing PST files on the network is not recommended or supported by Microsoft. However,
data stored locally on desktop or laptop computers is difficult to manage and makes archiving, backup, and deletion policies challenging to enforce. PST
Backup Attender is a lightweight, PST file backup utility used to locate, manage, and back up PST files on user computers, eliminating the need for storing
PST files on the network.
Capture Statistics and Generate Reports—The ability to report on the content of all PST files across the entire organization is an important
key step in understanding the information assets an organization possesses. With the use of Sherpa’s tools, gather comprehensive statistics and gain a total
perspective over your entire email environment, including all PST files, with information such as: total number of PSTs, total message/attachment counts,
size, and age.
DWORDs) are set to 0. These values live under the HKEY_CUR- your PST files under control.
RENT_USER\Software\Policies\Microsoft\Office\12\Outlook\Prefer-
ences\ key. Add them manually or using your favorite policy distri- PST files certainly have their drawbacks, and my long-standing
bution tool and users will no longer be able to use AutoArchive. recommendation is to avoid their use whenever possible. For many
organizations though, the flexibility and usefulness of PST files for
Auditing and Inventorying PST Files users outweigh their faults; so knowing how to effectively manage
One of the most important tasks in PST management is finding them can greatly ease your life as an Exchange administrator.
out where PSTs exist on the network. Sadly, Microsoft still does not
offer even a rudimentary tool for doing this. Third-party vendors Paul Robichaux is a founding partner at 3Sharp LLC, an MCSE, and an
such as Sherpa Software offer inventory and management tools Exchange MVP. He is the author of several books, including The Exchange
that can scan network shares and individual workstations for PST Server Cookbook (Published by O’Reilly and Associates); he is also an active
blogger at http://www.robichaux.net/blog.
files; you will need to be able to do such scans in order to bring
sponsored by The PST File Rodeo 7
