How to Block PST Files for the New Outlook for Windows

Use an OWA Mailbox Policy Setting to Block PST Access

A reader asked if it is possible to disable the ability of the new Outlook to open PST files, noting that some internet posts say that it’s not yet possible. One example from April 2025 points to the list of Microsoft 365 roadmap items related to PST files and suggests that support is coming.

Well, support is available through the OutlookDataFile setting in OWA mailbox policies. The default is to allow new Outlook clients to access PST files (a longstanding request for many people), but organizations that don’t want people to ever use PST files can easily block access.

Figure 1 shows the default access where users can add PST files for the client to open. As the note says, the current level of support extends to mail items only.

Good reasons exist to justify being able to open PST files. Access to old email is an obvious reason. eDiscovery investigations often use PST files to export mailbox items found by searches for review by external experts.

The downside of allowing access to PST files is the temptation for people to move items from mailboxes into PSTs. This action makes items invisible for compliance purposes. It also makes email inaccessible to AI tools like Microsoft 365 Copilot. More worringly, PSTs encourage bad behavior, such as people filling PSTs with email that they want to preserve when they leave a company. Using sensitivity labels blocks this habit because although users can keep protected items in a PST, they won’t be able to access the items if they can’t authenticate with an account that has access rights to the items.

Mailbox Policy Settings

An OWA mailbox policy is a collection of settings that govern how OWA works. Exchange Online supports multiple OWA mailbox policies, allowing administrators to create and assign different policies to user mailboxes.

The new Outlook is tightly linked to OWA, so it’s unsurprising to find that OWA mailbox policy applies to the new Outlook too, such as the setting to block downloading of attachments. In this case, OWA doesn’t support PST access at all, so the setting is unique to the new Outlook.

Block PST Access in the OWA Mailbox Policy

To block PST access, run the Set-OWAMailboxPolicy cmdlet to update the OutlookDataFile setting. This command updates the setting to Deny to block all access to PSTs:

Set-OwaMailboxPolicy -Identity OWAFullAccess -OutlookDataFile Deny    

The effect is shown in Figure 2. The Outlook Data Files option is now hidden.

Other values for the OutlookDataFile setting are:

• NoExport: Users can’t export from a mailbox to a PST.
• NoExportNoGrow: Users can’t export from a mailbox to a PST or copy items from a mailbox to a .pst file.
• NoExportNoOpen: Users can’t export from a mailbox to a PST, or open new PSTs.
• NoExportNoOpenNoGrow: Users can’t export from a mailbox to a .PST, copy items from a mailbox to a PST, or open new PST files.

These settings are the equivalent of the policy available to control PSTs in Outlook classic.

The effect of the new setting is not immediate. It takes time for Exchange Online to propagate the update to all the mailbox servers used by a tenant, and a further period before clients pick up and apply the setting. The time required might be as short as a few hours and as long as twelve hours. If you’re going to apply a block on PST usage, it’s best to implement the policy before people start to use the new Outlook.

PSTs are Ancient Baggage

Introduced in 1997, PSTs are an archaic part of the history of Exchange. Unfortunately, just like public folders (1996), customers can’t quite get rid of either. I guess we’ll just have to manage the beasts, which is what the mailbox setting described here does for PSTs in the new Outlook, aka Outlook designed for the 2030s dragging along ancient baggage…

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