The new Outlook for Windows is a sleek, fast, and web-powered replacement for the classic Windows desktop client. But for many organizations, the switch is a minefield of broken workflows. Before any wide rollout, IT teams must test three specific areas: PST-heavy mailboxes, COM add-in dependencies, and public-folder access. Skipping these checks risks user backlash, data loss, and support chaos.

Microsoft\u2019s push toward the new Outlook is unmistakable. Starting in 2024, the new experience ships as the default on Windows 11 devices, and the classic client is being nudged aside. Yet the company\u2019s own documentation lists a growing\u2014but still significant\u2014gap in essential features. The safe path is to pilot aggressively, identify incompatibilities, and hold back users who rely on unsupported functionality.

PST Files: Limited Read-Only Access and No Exports

Personal Storage Table (PST) files are a decades-old archive format for emails, contacts, and calendar items. Many organizations still use them for compliance, migrating legacy data, or simply as a dumping ground for old messages. The new Outlook\u2019s PST support is barebones.

You can open a PST file in the new client and browse its contents. That\u2019s it. You cannot create new PSTs, move items into a PST, export to PST, or modify existing ones in any meaningful way. For users who routinely drag emails into local archives, this is a showstopper.

The change isn\u2019t a bug\u2014it\u2019s a deliberate architectural shift. The new Outlook is built on Edge WebView2 and relies on Exchange Online\u2019s web protocols. PST files, with their deep integration into the classic MAPI subsystem, don\u2019t translate cleanly into this model. Microsoft\u2019s stated plan is to deprecate PSTs entirely in favor of cloud-based archiving, but no hard deadline is set.

For migration testing, target any user whose mailbox is larger than 50 GB and who has a habit of creating PST archivals. Look at storage reports for PST prevalence on local drives. Then test: can the user read their archived emails? Can they search effectively? If the answer is no, delay their switch until you have a cloud archiving solution in place or can train them to move data into shared mailboxes or In-Place Archives.

COM Add-ins: The End of the Line for Legacy Tools

Classic Outlook has supported COM add-ins for over two decades. These plugins are deeply hooked into the client, offering features like CRM integration, signature management, meeting room calendars, and custom email security. The new Outlook does not support COM add-ins at all. Period.

Instead, the new client runs web add-ins\u2014the same framework used by Outlook on the web. While the web add-in catalog has grown, many popular business tools have not been ported. Think of your organization\u2019s critical add-ins: Adobe Send & Track, Salesforce for Outlook, older legal compliance tools, and even legacy archiving solutions. Many of those will simply not appear in the new client.

Microsoft recommends switching to web-based equivalents or vendors\u2019 newer modern add-ins. However, that\u2019s not always possible. Some add-ins are custom-built for the organization and have no web counterpart. Others require tight integration with the Windows OS or network that web sandboxing prohibits.

Testing here requires an inventory of all COM add-ins in your environment. Use the classic Outlook\u2019s Trust Center to list them, or use a PowerShell script against the registry. Deploy the new Outlook to a few power users who depend on those add-ins. Monitor their support tickets. If a key add-in fails, the choice is stark: keep those users on classic Outlook until a replacement surfaces, or pressure the vendor to modernize.

Public Folders: A Half-Baked Portal

Public Folders, once the heart of Exchange collaboration, remain in use across thousands of organizations, especially for shared calendars, contacts libraries, and threaded discussion archives. The new Outlook\u2019s public folder support is frustratingly incomplete.

You can add a public folder hierarchy and view items, but the experience is clunky. Search often fails to index subfolders correctly. You cannot create new public folders, set permissions, or manage properties. Drag-and-drop into public folders may work sporadically. Mail-enabled public folders are even less reliable; messages can disappear without a trace. The classic \u201cFavorite Public Folders\u201d feature is absent. For teams that rely on public folders for daily workflows\u2014think a shared sales calendar or a customer-support ticket repository\u2014the new client breaks long-standing processes.

Testing for public folders means identifying the heaviest users. Look for public folder usage patterns via Exchange audit logs. Deploy a pilot and ask users to perform common tasks: find an item in a deep hierarchy, post to a discussion folder, update a shared contact. If frustration mounts, you have two options: keep those users on classic Outlook, or migrate the public folder content to modern alternatives like Microsoft 365 Groups, shared mailboxes, or SharePoint lists. Migrating public folders is a project in itself, often requiring months of planning and third-party tools.

How to Run a Successful Migration Pilot

The worst approach is to flip the switch organization-wide. Instead, follow a phased testing strategy that focuses on the three risk areas above.

1. Inventory your estate. Use tools like the Microsoft 365 Admin Center reports, Exchange Online PowerShell, and endpoint management data to find:

  • Users with PST files on their machines (scan common paths like C:\\Users\\*\\Documents\\Outlook Files\\).
  • A complete list of COM add-ins, including version numbers and vendor names.
  • The top public folder users by logon count.

2. Create test groups. Build a pilot group of early adopters who are technical and willing to cope with workarounds. But also include a small number of high-risk users from each identified category. Don\u2019t just rely on volunteers; you need real-world feedback from the people who will be most impacted.

3. Document workarounds. For PSTs, guide users to open their PST in classic Outlook if they need occasional read access, or move archives to a shared mailbox. For COM add-ins, research web alternatives or vendor roadmaps. For public folders, create a cheat sheet showing how to access them in the new client\u2014and note what\u2019s broken.

4. Set clear \u2018stay on classic\u2019 criteria. Define objective conditions that mandate a user remain on classic Outlook. Example criteria: user has one or more COM add-ins critical to their role; user accesses public folders with more than 50 subfolders daily; user has PST archives larger than 10 GB that they modify regularly. Communicate these rules to the help desk so they can handle requests efficiently.

5. Monitor and iterate. Collect feedback via surveys or direct interviews. Track help desk tickets tagged \u201cnew Outlook.\u201d Update your readiness assessment as Microsoft releases updates. The feature gap is narrowing with each month, so a user who fails today might pass in six months.

What Microsoft Still Needs to Address

Microsoft\u2019s roadmap for the new Outlook hints at some improvements: partial offline support (already available in preview), better public folder performance, and a migration tool to move PST data to the cloud. But no timeline is confirmed. The uncertainty leaves IT pros in a bind\u2014caught between the eventual deprecation of classic Outlook and the new client\u2019s missing pieces.

The official classic Outlook client will remain supported at least until 2026, according to recent Microsoft statements, but extended support may continue beyond that. Organizations should use that window to modernize their email workflows: retire COM add-ins, move PST data to Exchange Online archives, and transition away from public folders. The new Outlook is the future; it\u2019s just not ready for everyone today.

The Bottom Line

Don\u2019t rush the new Outlook migration. For many companies, it\u2019s a step forward in performance and security. For others, it\u2019s a step off a cliff. By testing PSTs, COM add-ins, and public folders first, you\u2019ll know exactly where you stand. Arm yourself with data, create a solid fallback plan, and let Microsoft mature the product before the mandatory switch flips.