New Outlook for Windows: What Businesses Do Before March 2027

Microsoft starts pushing businesses off classic Outlook in March 2027. That's when the opt-out phase begins for commercial Microsoft 365 tenants, which means users get switched to new Outlook by default and have to choose to go back. You have about five months to find out what breaks before Microsoft finds out for you.

Classic Outlook isn't going away yet. But the default is about to flip, and the businesses that test first will have a quiet spring.

When does new Outlook become the default for businesses?

The opt-out phase for commercial and GCC tenants starts in March 2027. Microsoft moved it from April 2026 in a February 20, 2026 update to Message Center post MC949965, which lists an "act by" date of March 1, 2027. GCC High and DoD dates come later.

There are three stages: opt in, opt out, then cutover. We're in opt in now. Microsoft hasn't dated cutover.

Here's what Microsoft has committed to in writing on its product availability page:

• "Existing installations of classic Outlook through perpetual and subscription licensing will continue to be supported until at least 2029."
• Admins get "at least 12 months of notice before the cutover stage is implemented in Production rings."

So nobody loses classic Outlook in 2027. What changes is who has to make the effort: today a user opts in to new Outlook, and from March 2027 they'll have to opt out.

What breaks when you switch to new Outlook?

Add-ins and macros break first. Microsoft says it plainly: "In new Outlook for Windows, COM add-ins aren't supported," per its new Outlook overview. Its add-in guidance adds: "New Outlook for Windows doesn't support VBA."

That matters more than it sounds. COM add-ins are how a lot of line-of-business software plugs into Outlook:

• Document management. The "file this email to the matter" button many law and accounting firms rely on.
• CRM. Older connectors that log emails to a contact record.
• E-signature, archiving and e-fax tools that put a button on the ribbon.
• Homegrown VBA macros someone wrote in 2014 that the office manager still runs every Friday.

Web add-ins do work. Microsoft says "existing web add-ins are fully supported, with no extra work required from partners." So the question for each tool is simple: does the vendor ship a web add-in yet?

Two more gaps worth checking.

Microsoft's migration planning guide says "Exchange on-premises is currently unsupported," and new Outlook needs an internet connection.

If you still run an Exchange server, start there. Our guide to Exchange Server end of support covers the move.

Who won't be switched automatically?

Some users stay put. MC949965 says the automatic switch skips:

• Users opted out through the admin-controlled migration policy
• Users whose toggle is hidden by policy
• Users on a perpetual license, such as Office 2021 or 2024
• On-premises mailboxes

Microsoft also says "existing policies that disable or limit access to new Outlook will remain in effect." If you already hid the toggle, nothing changes in March.

Don't treat that as the plan, though. Blocking buys time. It doesn't fix the add-in your billing team can't work without.

What should a small business do before March 2027?

Run a pilot now, on your schedule. Microsoft recommends a pilot of at least 45 days, so a pilot that starts after the holidays still finishes before March. Here's the order we use.

1. Inventory your add-ins. The Microsoft 365 Apps admin center has an add-in inventory. On a single PC, it's File, Options, Add-ins. Write down every COM add-in and every macro.
2. Call your vendors. Ask each one: do you have a web add-in for new Outlook, and does it do everything the old one did? Get the answer in writing.
3. Pick a pilot group. Five to ten people across departments, including your heaviest Outlook user. Microsoft suggests starting with new hires and the people piloting Copilot. If you're weighing Copilot, our Copilot licensing FAQ covers the plans.
4. Move them with the admin-controlled migration policy. You can scope it to an Entra ID group, and users can switch back. That's a safe test, not a cutover.
5. Keep a list of what's missing. Shared mailboxes, delegate calendars, signatures, rules, search. Check each against Microsoft's feature comparison on the day you test, because it changes month to month.
6. Decide per team. Teams with no add-in problems can move early. Teams waiting on a vendor stay on classic Outlook, with the toggle hidden until the vendor ships.

If you're still on Office 2021, this isn't your biggest problem. It's out of support. Read Office 2021 end of support first.

Is new Outlook actually better?

For most people, it's fine. It looks like Outlook on the web because it's built on it, and it's where Microsoft is putting its new work.

For power users, it's a downgrade today. No COM add-ins, no VBA. Plenty of firms will run both clients for a while, and that's a reasonable answer.

What won't work is ignoring it. In March 2027 someone in accounting clicks through a prompt, loses the button they file invoices with, and calls you at 4:45 on a Friday.

Frequently asked questions

Is Microsoft getting rid of classic Outlook?

Not yet. Microsoft says existing classic Outlook installations on perpetual and subscription licenses will be supported until at least 2029. What changes in March 2027 is the default: users get moved to new Outlook and have to opt out to go back.

Can we stop the switch to new Outlook?

Yes, for now. Admins can hide the toggle in classic Outlook, and Microsoft says existing policies that limit new Outlook stay in effect.

Users who are hidden from the toggle aren't moved automatically. Use that for teams waiting on a vendor, not as a permanent answer.

Do Outlook add-ins work in new Outlook?

Web add-ins do. COM add-ins and VBA macros don't.

Check every add-in your team uses, then ask the vendor whether a web version exists and whether it matches the old one. Document management, CRM and e-signature tools are the usual problems.

Does new Outlook work with on-premises Exchange?

No. Microsoft's planning guide says Exchange on-premises is currently unsupported in new Outlook, and on-prem mailboxes aren't migrated automatically. If you still host your own Exchange server, moving that mail to Microsoft 365 comes first.

How long should a new Outlook pilot run?

Microsoft recommends at least 45 days. That's long enough to hit a month-end close, a billing cycle and the odd workflow someone only runs once a month. Start with a small group from each department, including your heaviest Outlook user.