Why you shouldn’t use PST files

5 12 2009

They have been around for years and for thousands of Microsoft Outlook users and email administrators out there, they’d be lost without them: Personal Storage Table (PST) files. If you’ve worked with Outlook for very long, the name will immediately ring a bell; if you’ve ever administered Outlook, you may already know about the problems associated with this notorious file format.

In any corporate environment – or, for that matter, any environment with an Exchange Server – the use of PST files as a permanent solution to an email administrator’s problems should be banned. Let’s find out why.

Problem 1: File Sizes and Data Security

The number one issue with the PST format prior to Outlook 2003 was that it was ANSI (American National Standards Institute)-based. The ANSI PST format has a maximum size limit of 2GB, and other limitations exist with regard to the number of items which can be stored per folder. However, there was a particularly problematic bug in the Outlook software which allowed data to be written to ANSI PSTs past the 2GB limit without warning. This would result in data loss, at least past the 2GB limit, but potentially loss of all the data stored in the file.

To address these concerns, Outlook 2003 and higher introduces a new PST format which runs on Unicode instead. This format stores up to 20GB of data, but it should be noted that upgrading Outlook does not automatically upgrade any PST file(s). This must be completed manually, by creating a new Unicode file and transferring the data across.

Despite the improvements made, PST files are still susceptible to corruption issues – which will result in lost data. These become particularly prevalent as files become larger or you increase the volume of data which moves through the file. For most users, the prospect of losing precious or business-critical emails, reminders, tasks and contacts could be cause for significant concern. It shouldn’t come as a surprise that you should make a regular backup of your PST file(s), but this is not completely safe, as a PST can go for weeks or months in a partially corrupted state before you realise you have a problem.

Problem 2: Network Access and Backups

PST files must be stored on a local hard disk. Accessing them over a network is not supported by Microsoft. Instabilities in the network, loss of network connectivity, speed issues in reading and writing from the file server can all cause issues — particularly for sensitive PST files, which are so very easily corrupted.

This has two implications for system administration:

Firstly, backups are already difficult to maintain, due to the issues with corruption going undetected, but will become ever more difficult to implement. As the PST cannot be run from the network, you must configure backups on each machine individually – and must ensure the backup does not run while Outlook is running. Backing up the Exchange Server is rather pointless, as the data is offloaded into the PST when the user logs in.

Second, your cost of administration increases significantly. Considering a typical organisation, which may have remote workers and several sites across different areas of the country or perhaps throughout the world, moving administration away from the server and towards the client lessens the design principles surrounding central administration, requiring more admin time to perform repetitive tasks on PST files. The system may quickly grow beyond your control, becoming exponentially difficult to track and maintain.

Problem 3: File Sharing and Remote Access

PST files do not natively support file sharing between multiple users simultaneously. If you attempt to configure this, the mail file may be corrupted — not to mention the fact you would need to run the file over the network, so problem #2 has already been invoked.

Storing data in PST files also has no benefits for remote access either. Exchange’s Outlook Web Access (OWA) (or Outlook Web App, in Exchange 2010) allows users to remotely access their mailboxes, providing a near Outlook user interface for doing so. Data in PST files has usually been removed from the mailbox, so immediately becomes inaccessible to the user remotely.

Problem 4: Inefficient use of resources

You’ve invested in a powerful Exchange Server. It: has large, redundant disk arrays, processing power and RAM capacity; cost you thousands to purchase the hardware and software licenses; adds significantly to your energy and data centre cooling bill. If PST files are in use, your server is essentially going to waste; the functionality of the server you are actually using is essentially the same as a free Linux mail server distribution running on an old workstation supporting POP3 clients.


Despite the considerations above, you might still be wondering how to work around those common problems which PST files are oh so convenient for solving.

Use 1: Archiving

This is a mis-conception, brought about largely by Outlook’s desire to continue annoying its users with AutoArchive prompts. There is no reason whatsoever that mail should be archived to each user’s local PC. Consider the actions you would take to archive files off your file server; where would you put the archived data? On your own PC? On your manager’s? On the CEO’s? You’d do none of those three, as the data is unlikely to be backed up, and you cannot assure data security. Instead, you’d find some space on a share on your archive server – or create a LUN using spare space on one of your SANs.

The same applies to email. Off-loading email from your Exchange Server to user PCs has significant risks attached to it. Instead, you should use an enterprise mail archiving solution. The product I usually recommend is Symantec Enterprise Vault, although there are many others. The main benefits? Data is still stored centrally, under the guise of your retention policies and backup process. To the end user, they can still view archived emails using a handy web interface (yes, a web interface – providing remote access to the archive).

Okay, but what about when disk space on my Exchange Server runs low or I hit the store size limit?

UPGRADE THE SERVER! Exchange 2007 and 2010 do not impose a hard limit on the mail stores, and you shouldn’t be trying to run a mail server with little disk space or database space remaining. Archiving to PST is a quick solution, but one which won’t work in the long run.

With the soon-to-be Exchange 2010 release, significant changes have been made, one of which is the addition of archiving support. Each user can be given a separate ‘archive’ mailbox; it is attached to their main mailbox, but allows for data to be archived for long term storage. The settings governing when and how mail is moved to the archive are controlled by retention policies, giving the administrator greater control over retention. Again, the archive store is available remotely via Exchange 2010’s Outlook Web App.

Use 2: On the road

For users on the road, there is no need to store their mail in a PST file. Cached Exchange Mode is available in Exchange 2003/Outlook 2003 and higher, allowing users to work offline with a cached copy of their mailbox. When they reconnect to the network, the changes are seamlessly synchronised back to the server.

Use 3: Exmerge/Export-Mailbox

This is just about the only use of PST files which I can agree to — and I’ll admit, I’ve used this approach myself. If you migrate to a new mail system or rebuild your Exchange system, sometimes you cannot avoid using exmerge (or Exchange 2007’s export-mailbox management shell cmdlet) to take handy copies of the mailboxes – which can later be re-imported to the new system. For moving mailboxes between servers, you would use the Move Mailbox wizard – but for large scale rebuilds, exmerge is sometimes your friend.

Be cautious though; Exmerge uses the ANSI PST format, so you will need to meticulously plan your export and import procedure for larger mailboxes.

Use 4: Home Users

These are the people who the PST is most applicable to. If you are connecting via Outlook to a Post Office Protocol (POP) host to download your email, that email will be stored in a PST file. The fact you don’t have an Exchange Server doesn’t change any of the points above, though; that PST is still susceptible to corruption. If mail is deleted off the server, this could lead to data loss.

For this issue, you really have two solutions. The POP3 account in Outlook can be configured to leave email on the server. This acts as a backup; if your PST file becomes corrupted, the ISP still has a copy of your messages, so they can be downloaded again. To configure, open the Tools > Account Settings dialog in Outlook. Select your POP3 account, choose Properties, press More Settings, then switch to the Advanced tab. Under the Delivery section at the bottom of the window you should check the “Leave a copy of messages on the server” checkbox. If you want a backup of all your mail, don’t enable the option to remove it from the server after a certain time period.

The disadvantage to the POP3 solution becomes apparent if you move to another computer or access your mailbox via your ISP’s webmail interface. The message state information (tracking of read/unread or whether the message has been replied to or forwarded) is not transferred back to the ISP, so all the mail you thought you had read and handled will still be marked unread on the ISP’s server.

My preferred solution, and the one I use regularly, is an Internet Message Access Protocol (IMAP) account. The IMAP protocol is another mail protocol used to access email; it stands alongside POP. However, using IMAP, you replicate a client-server topology very similar to connecting to an Exchange mailbox with Outlook in Cached Exchange Mode. With IMAP, email generally remains stored in your mailbox at the ISP until you specifically delete it. Nevertheless, you can’t get away from PST files completely; they are still there when you use an IMAP account, as Outlook uses them to make a cache of the data for working with the IMAP account in offline mode. However, as the PST isn’t the only location where your data is stored, any corruption is not going to lead to data loss.

It should be noted that both the POP solution for leaving data on the server, as well as the IMAP solution, both have drawbacks, as items in your Calendar, Contacts or Tasks folders will not be stored on the server. IMAP does not support special folders – such as the Calendar or Tasks – and these will not be replicated back with a POP account, so you will still be using a PST file to some extent. Unless you move entirely into the cloud (use web services for email, calendar and contacts) or purchase your Exchange Server, you won’t be able to easily get away from this.


I’ve covered a fair bit of information regarding PST files here. Hopefully, my points detailing why the use of PSTs is so impractical will now encourage you to reconsider your PST usage, archiving practices and retention policies.

With all your user mail stored safely on the Exchange Server, rather than local PCs, assistants can become delegates for their managers, looking after their mailbox; the administrator can rest assured that all data is centrally stored and backup up and you can turn off Outlook AutoArchive, relieving end users of that annoying prompt every couple of weeks.

This article was originally published at Experts Exchange.