Classic Outlook is retiring: the key dates, and the one that is weeks away

If you open Outlook every morning without a second thought, here is one worth two minutes of your time. Microsoft is in the middle of replacing the classic Outlook for Windows that most businesses have used for years with a completely new app. It is not a single switch-off. It is a staged, multi-year move, and Microsoft has already pushed the dates back more than once.

That makes it easy to ignore. But one part of this change has a deadline only weeks away, and it can quietly break things you rely on. Here is the plain-English version of what is happening, what actually affects you, and what to do about it. If you would rather we just checked it for you, book a free IT review.

Does any of this sound like you?

• You are not sure whether your team is on classic Outlook or the new one.
• You have a backup, archiving, CRM or line-of-business tool that connects to your mailboxes.
• You rely on an Outlook add-in that someone set up years ago.

If so, read on.

The key dates for classic Outlook

Microsoft is moving everyone from classic Outlook to the new Outlook for Windows over several years. The headline milestones:

• Now: the new Outlook became the default for Microsoft 365 business and enterprise users in April 2026. You can still switch back to classic for the time being.
• Maintenance mode: classic Outlook still works and still gets security updates, but new features are going into the new app, not the old one.
• Around 2028: the cutover phase, when classic Outlook starts being removed from the standard Microsoft 365 apps and loses some cloud connectivity for most users.
• 2029: Microsoft has committed to supporting classic Outlook, including security updates, until at least 2029 for Microsoft 365 subscriptions and Office LTSC perpetual licences. After that, no more patches.

Our honest advice: treat 2029 as your planning horizon, not a promise. This timeline has already slipped more than once, so plan for the direction of travel and keep an eye on the official Microsoft 365 roadmap rather than banking on an exact date.

The deadline that bites first: EWS, 1 October 2026

Underneath the Outlook headline is a nearer, sharper deadline that has nothing to do with the app on your desktop. It is about how other software talks to your Microsoft 365 mailboxes.

Exchange Web Services, or EWS, is an older way for third-party tools to connect to Exchange Online. Think backup and archiving products, CRM and helpdesk integrations, email signature tools and various line-of-business apps. From 1 October 2026, Microsoft starts blocking EWS requests from non-Microsoft apps unless your tenant has specifically allowed them, and EWS is removed for good on 1 April 2027.

In plain terms: if you have a tool that still uses EWS and it is not on the allow-list, it can stop working from the start of October. That is the part most likely to catch a business by surprise, because the tool keeps running fine right up until the day it does not.

Two more things worth knowing

• Add-ins do not automatically carry over. Many add-ins built for classic Outlook need to be rebuilt for the new Outlook. If your team depends on one, it is worth checking now whether a new-Outlook version exists.
• The new Outlook is genuinely different. It is built around Exchange Online and cloud services, so how it handles some settings, shared mailboxes and offline access is not identical to classic. Worth a short test before your whole team moves.

What to do about it

You do not need to panic or rush your whole team onto the new app tomorrow. But two things are worth doing now:

1. Check for anything using EWS before 1 October. Ask us, or your software vendors, whether any of your integrations still rely on EWS, and get them updated or allow-listed in good time.
2. Plan the move to the new Outlook. Test it with a small group, check your add-ins and shared mailbox workflows, and roll it out on your terms rather than being moved automatically.

Why we are flagging this

We have supported UK businesses since 2006, and the changes that cause the most disruption are rarely the ones with the big headlines. They are the quiet, back-end deadlines like EWS that no one notices until a familiar tool stops connecting. Part of our job is to see those coming for you.

Want us to check whether any of this affects your business? Get in touch or call us on 01452 701355, and we will take a look. Prefer to talk it through first? We are happy to.