Export to PST with the New Outlook for Windows is Slow and Maybe Shouldn’t be Allowed

Giving the New Outlook for Windows (Monarch) client the ability to deal with PST files has long been one of the biggest demands from people who don’t want to move from Outlook (classic). Microsoft delivered code to support import access for PST files a few months ago. Now code to export mailbox items from the New Outlook to a PST file is available in targeted release tenants (message center notification MC1104309, 26 June 2025, Microsoft 365 roadmap item 485737). General availability is scheduled for mid-July 2025. GCC tenants receive the update about a month later.

The Dire Slowness of Export to PST

Before we all get too excited, let me report that exporting mailbox items from the New Outlook to a PST is a slow operation. It took Outlook 27 minutes to export 4,829 items from my Sent Items folder to a PST. The eventual PST ended up as a 1.01 GB file. Your mileage might vary depending on network speed, current demand on the service, and PC configuration, but I doubt that the overall result will be much different. By comparison, exporting the same folder from Outlook (classic) took less than two minutes.

The reason for the slowness is simple. The New Outlook needs to do a lot of work to extract each item from the source mailbox and convert it to the format used by PSTs. It’s not like Outlook (classic), where PST support has been incorporated into the client since day zero and the client and PST use the same MAPI-based underpinnings.

PST and OST files are very close in structure and format. Outlook (classic) uses OST files for offline synchronized copies of mailbox folders and items. The New Outlook takes a completely different approach to offline access, and the differences in approaches contribute to export slowness.

But don’t worry, Microsoft has an option coming to schedule a mailbox export (Microsoft 365 roadmap item 485743) that should remove the pain of watching the New Outlook slowly export items from mailbox to a target PST (Figure 1). The scheduled export option is due to arrive any day now and should help.

Export to PST Limited to a Mailbox or Single Folder

The new Outlook supports the export of a complete mailbox or a selected folder (Figure 2) for any of the mailboxes configured in the Outlook profile. I’m a tad baffled by the design decision to limit export to these options. Exporting an entire mailbox is fine, but why allow the choice of just one folder?

It would make more sense to allow the selection of multiple folders. In Figure 1, the Inbox folder is selected, but I might want to include the Calendar, Sent Items, and other folders in the target PST. Apart from mimicking the process used by Outlook (classic), there doesn’t seem to be much reason to restrict an export to one selected folder.

Do You Want People to Use Export to PST?

The big question remains do you (or rather, the organization) want to allow Outlook users to be able to export items to a PST. It depends on the compliance and governance strategy for the tenant. If you want everything stored in Microsoft 365 to allow Purview solutions like eDiscovery or AI solutions like Microsoft 365 Copilot to be able to find everything, then it seems like allowing people to export items is a bad idea. All you’re doing is giving people an invitation to move messages out of sight. Users might want to export their mailbox, especially when they’re about to take up a new job in a different company, but that’s no reason to allow mailbox exports to happen.

Even with a strict compliance regime in force, there will be situations when PST exports are justified, such as providing copies of items to external experts for review. However, that’s still not a reason to permit mailbox exports across the board.

Fortunately, blocking access to PSTs is easy and quickly accomplished with an update to OWA mailbox policies. Figure 3 shows the result. New Outlook can’t open a previously-added PST and the Export option has disappeared. I like it this way…

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