Many users have Outlook configured to use a Personal Folders file (PST) rather than an Exchange server mailbox. While anyone using a POP3, IMAP, or SharePoint links must use a PST, many Exchange server users choose a PST for various reasons, most often because they have a very small mailbox quota. While PST usage with Exchange accounts is counter to many of the reasons for using Exchange server, many organizations continue to allow the use of PST files.

Exchange administrators often say that PST files are "bad". What they are really saying is that a PST file isn’t the best choice as a storage location for Exchange mailbox data for several reasons. Most importantly, PST files aren’t supported for over the network access. While there is an increased risk of corruption with network storage, they may not get backed up regularly if kept on the local drive. Unlike mailbox folders, folders in a PST can’t be shared with other network users or accessed from other computers or using OWA. PST files are much less secure than a mailbox as passwords can be easily removed with readily available tools. Finally, contents of a PST are larger than in an identical Exchange mailbox, both because they don’t support single instance storage and because they contain TNEF data.

When a user needs a local copy of their mailbox, they should use cached Exchange or offline folders. This allows them to take full benefit of OWA, share folders and have a local copy of their mailbox.

PST files do make quick, inexpensive archives, at least until you need to search them for compliance and discovery. For this reason alone, a centrally managed archive is a better option

Networking PST files

We get too many questions from users and administrators who are accessing PST files over the network or who want to. This is not recommended and is not supported by Microsoft. While there are many things we do that are "not supported by Microsoft", in this case data loss can result.

Yes, they often work fine, at least until the network goes down and access is cut off in the middle of an update to the data file. If you are storing PST files on the network to insure they will get backed up, keep a copy of the PST on the local drive and copy it to the network when Outlook is closed.

Personal folder files are unsupported over a LAN or over a WAN link
https://support.microsoft.com/kb/297019

Network Stored PST files ... don't do it!
https://blogs.technet.com/askperf/archive/2007/01/21/network-stored-pst-files-don-t-do-it.aspx

Unlocking a PST

An administrator had this question about the use of Distributed File System and PST:

We are implementing DFS and want to reduce the file lock on PST’s. All users connect via Citrix. I have seen reference the following reg key "PSTDisconnectDelay” to alter the timeout value on the file lock. However, I can not find the \pst reg key on any server. Should the key be there already?

No, in most cases, the PST key does not exist so you will need to create it. Note that DFS is not recommended for PST files that are accessed across the network. It will work fine for PST files that are archived on the network drive, with the PST copied to the workstation before opening.

The registry key you'll need to add is (replace the xx.0 with your Outlook version number). (replace the xx.0 with your Outlook version number):

HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\PST
DWORD: PSTDisconnectDelay
Data Value: hexadecimal value where 10 equals 16 seconds

For more information see:
Distributed File System Replication: Frequently Asked Questions
“Can DFS Replication replicate Outlook PST files?”
https://technet.microsoft.com/en-us/library/cc773238(WS.10).aspx#BKMK_050

How to Change File LockTimeout Value for PST Inactivity
https://support.microsoft.com/kb/222328

Storage Media and PST Files

Since network use is not recommended, what storage media, besides the local hard drive, is safe to use for PST files?

Floppy disks are not PST-friendly; fortunately very few people use floppy disks anymore. Their read/write speed is too slow for Outlook (and many other applications), leading to many corrupt and unrecoverable files.

USB drives are fast enough but you need to remember to close Outlook before removing the stick. Doing so can corrupt the PST. For best results, wait several minutes after closing Outlook before ejecting the USB to insure Outlook is closed and the lock is released. SD and other flash cards are also fast enough. As with USB, do not remove the media while Outlook is open.

CD/DVD are fine for storing large archive files but Outlook needs read/write access to the PST, so while CD/DVD drives are fast enough, only CDRW and DVD/RW support read/write access which Outlook requires. If you save an archive PST file to a CD or DVD, you'll need to copy the PST back to your hard drive to read it (and remove the Read only flag if it’s enabled).

Recovering an Orphaned OST

Is there any way to recover an .ost file that was orphaned after an exchange server was decommissioned?

If the OST is from an older version of Exchange, the free OST2PST utility should work just fine. However, it does not support Unicode OST format. For those you'll need a commercial recovery utility.

If the Exchange account was recently removed from Outlook and you use System Restore you may be able to recover the account and export the OST to a PST file.

Convert an Exchange Offline file (OST) to a Personal Folders file (PST) (includes a link to OST2PST download and commercial utilities.)

ANSI and Unicode PST Formats

If you use Outlook 2002 or earlier or are using a PST created in Outlook 2002 or older, you are using an ANSI format PST. This has a limitation of 2 GB. Outlook 2003 and 2007 support Unicode format, with a PST size of 20 GB by default. They support up to 33 TB, which may be practical some day. When you upgrade to Outlook 2003/2007, you need to make new PST files, Outlook will not upgrade a PST to the new format.

Unicode PST formats offers other benefits besides larger file size. Each folder can contain unlimited numbers of items (ANSI maximum is 65,536 items).

One important limitation of the Unicode PST format is that they can't be read by Exmerge as it only supports ANSI format.

Error message when you use the ExMerge tool to export an Outlook 2003 mailbox to a .pst file: "The item could not be moved"

Convert an old PST to a Unicode PST