Managing Personal Folders (.pst files) remains a top concern for Exchange administrators and Outlook users. Periodically, I like to answer your questions about .pst files. Previous installments of my .pst file Q&A sessions are available at the URLs below.

Q. It was news to me that .pst files have a 2GB limit. Do .ost files have a similar limit?

A.Yes, .pst files and .ost files share the same architecture and so are subject to the same limits--about 1.8GB in the ANSI-format .pst files and .ost files compatible with Outlook 2002 and earlier versions. Office Outlook 2003 supports .pst and .ost files as large as 33TB, with a default maximum of 20GB. For more information on this newer format, which is also more resilient and supports Unicode data, see "Dealing with .pst Files," ( ).

Q. How do I open a .pst file that someone sends me as an email attachment? Outlook displays a warning that it has blocked access to the file.

A. Outlook considers a .pst file to be an unsafe file attachment. The Microsoft article, "Cannot open attachments in Microsoft Outlook" ( ) explains how to modify the list of blocked file types.

Q. Is there a command-line tool I can use as an administrator to compact .pst files from the server console?

A. Microsoft provides no command-line method to compact a .pst file and recover the space from deleted items. However, if the PC is idle (with no screen saver program running) and there is enough space worth recovering, Outlook will perform a compact operation in the background. The Microsoft article, "How to compact the personal folders (.pst) files in Outlook 2002" ( ) explains that compaction runs if a file has 16KB space that has been freed since it was last compacted.

As an interesting exercise, you can check the .pst file size on disk, then delete a bunch of Outlook messages, empty the Deleted Items folder, and walk away from the machine for half an hour. When you come back, you should see that the file size is significantly smaller because Outlook has compacted it.

PSTCompactor ( ) and PSTCompress ( ) are two centralized tools for compacting users' .pst files. They also compress attachments inside Outlook items into .zip file attachments to create more free, recoverable space.

Q. How can I update the AutoArchive location for Outlook 2003 users who are using Exchange mailboxes? I need to update a few thousand users quickly and change the location from the default path to a network drive.

A. The Microsoft article "How to set the location of an archive .pst file in Outlook" ( ) explains how to create and use an Outlook profile (.prf) file to update the email profile settings to use a different default archive file. However, putting it on a network drive is a bad idea. (See next question.)

Q. You wrote in an earlier article that "Outlook doesn't support storing .ost or .pst files on network drives." We have about 20 Outlook 2003 users that operate from .pst files. Does your statement mean that storing the files on network drives doesn't work or that you can't use UNC paths to load .pst files?

A. Placing a .pst file on a network drive is an unsupported configuration. Outlook doesn't prevent you from doing so, but Microsoft warns that not only will access be significantly slower, but the likelihood of corrupting the .pst file is high. The article "Personal folder files are unsupported over a LAN or over a WAN link" ( 297019 ) discusses this topic.

Q. How can I permanently change the display name for my .pst archive files from "Archive Folders" to something more specific? I now have several .pst files in the Folder List with the same Archive Folders display name. A. If a .pst file is still actively in use as an archive (i.e., referred to by either the default archive settings or any per-folder archive settings) Outlook will automatically rename it to Archive Folders. I hate that, too, but I don't know any way to thwart the built-in behavior. You might want to check all the folder settings to make sure that all folders are archiving to one current archive file. Or, if you really do want to have multiple active archive .pst files with different display names, turn off AutoArchive and move items into those folders manually.

Outlook 2007 Security Changes External applications that automate Outlook will no longer trigger security prompts on systems with up-to-date antivirus protection, beginning with Outlook 2007 Beta 2. According to Outlook programmability program managers Randy Byrne and Ryan Gregg, in their video "Developing Solutions Using the Consolidated Outlook 2007 Object Model" at , Outlook will automatically turn off the "object model guard" if it detects that the computer is running an up-to-date antivirus program. In network environments, they say, administrators will be able to control the security prompt behavior through registry settings, as well as through the older method of using a special Exchange public folder. The video also includes detailed demos on several of the new programmability features in Outlook 2007: rules, search, context menu, and custom form regions.

Your .pst File Questions Answered

Common .pst File Questions