Outlook Office 365: Which Version You Have and What's Changing

“Outlook Office 365” is the Outlook email and calendar app that comes with a work, school or personal Microsoft 365 (formerly Office 365) subscription. You can use it in a browser, as a desktop app on Windows or Mac, or on mobile. As of November 2026, the main thing Windows users need to know is that two desktop apps exist side by side, classic Outlook and new Outlook, and Microsoft has pushed back the date it will start making new Outlook the default in businesses.

This guide covers the naming, how to sign in to the right place, how the two Windows apps differ, and what the current timeline looks like.

Office 365 vs. Microsoft 365: what the name means now

On April 21, 2020, Microsoft renamed its consumer and small business Office 365 plans to Microsoft 365. Some enterprise plans kept the Office 365 name. That’s why people still search for “Office 365” when they mean Microsoft’s Outlook-plus-Exchange email service.

What matters in practice is which plan you’re on. Secondary sources report that some business plans, such as Business Basic, include web and mobile Outlook but not the desktop app. I couldn’t confirm current plan contents from a primary source, so check Microsoft’s plan comparison page or ask your admin if you’re not sure whether you’re licensed for desktop Outlook.

Signing in: work account or personal account?

The most common Outlook Office 365 problem is signing in to the wrong service. There are two, and each takes a different account type.

Microsoft’s sign-in guide for Outlook on the web says Outlook on the web is the browser version organizations use for work or school email, calendar and contacts. Personal accounts go through Outlook.com instead. If you get an error saying you can’t sign in with a personal account there, Microsoft’s advice is to sign out and sign in again with the correct account.

Some universities have told staff that Microsoft is removing the option to link personal and work mailboxes inside Outlook on the web during 2026. The desktop apps still support multiple accounts. If you relied on mixing accounts in the browser, check with your IT team.

Classic Outlook vs. new Outlook for Windows

On Windows, you’ll find two apps. The older one shows up in the Start menu as “Outlook (classic)”. The newer one is just “Outlook”, and it’s built on the same foundation as Outlook on the web.

Reviewers, including Windows Latest, still describe new Outlook as missing some classic features. The most common blocker is the loss of the older development technologies that COM add-ins and macros depend on. If your team relies on a CRM plug-in, a document management add-in or custom VBA, test it in new Outlook before switching anyone over.

How to try new Outlook

Microsoft’s switching guide describes the official route:

1. Open classic Outlook.
2. Select the Try the new Outlook toggle in the upper-right corner.
3. Wait for the new app to download. When it’s done, classic Outlook closes and new Outlook opens.
4. Sign in and choose whether to import your accounts and settings.

How to switch back to classic Outlook

Microsoft says you can switch back at any time:

1. In new Outlook, select the toggle at the top right. On some versions, you can also go to the Help tab and choose Go to classic Outlook.
2. When the dialog asks, choose the option to switch back to classic Outlook for good. Third-party guides say there’s also a one-time option, which means a later launch may put you back in new Outlook.
3. To stop Outlook from moving you automatically, open classic Outlook and go to File > Options > General > New Outlook options. Then clear Automatically switch me to new Outlook.

If the toggle is missing, Microsoft lists three likely causes. Classic Outlook may not be installed. You may have opened “Outlook” rather than “Outlook (classic)” from the Start menu. Or your organization may have made new Outlook permanent for your account. In that last case, only your IT helpdesk can change it.

The rollout timeline (as of November 2026)

Microsoft is moving business customers to new Outlook in phases. The dates have shifted, and older articles still list April 2026, so use this summary, which is based on reporting from Office Watch and Windows Latest about Microsoft’s Message Center post MC949965:

• Opt-in (now): Users can choose to try new Outlook.
• Opt-out (from March 2027): New Outlook becomes the default, but users and admins can still switch back. This phase was originally planned for April 2026.
• Cutover (no earlier than March 2028): Reports say managed customers will get at least 12 months’ notice before this final stage.
• Classic Outlook support: Sources agree on 2029 but word it differently. Some say “at least 2029”, some say April 2029, and TechRadar has reported that Microsoft hasn’t published a firm end-of-support date. Office Watch points out that the change to “at least 2029” is significant.

If your admin has already set the migration policy to keep users on classic Outlook, or has hidden the new Outlook toggle, the default switch shouldn’t affect you. These dates come from Microsoft’s own announcements, which have changed before, so admins should check the Microsoft 365 Message Center for the latest version.

The classic theme for new Outlook

To make the move easier, Microsoft added an optional style that makes new Outlook and Outlook on the web look more like the classic app. According to Message Center post MC1458476, it was scheduled to roll out worldwide between August and October 2026, so most users should have it by now.

To turn it on:

1. In new Outlook or Outlook on the web, open Settings (the gear icon).
2. Go to General > Appearance.
3. Select the classic Outlook style.

If you don’t see the option, the rollout may not have reached your account yet. It’s off by default, but Microsoft is turning it on for some users who move over from classic Outlook. As Office 365 for IT Pros and others note, it only changes how Outlook looks. It doesn’t add missing features, it doesn’t start a migration, and while it’s on, other theme options such as image backgrounds and accent colors are unavailable.

Classic Outlook is getting its own visual refresh too. Message Center notes say the update reaches Current Channel from early November 2026.

What to do now

• Individual users: If new Outlook covers your everyday email and calendar needs, it’s fine to use. If something you rely on is missing, switch back. That’s officially supported.
• Team leads and admins: List every add-in and macro your team uses and test each one in new Outlook before March 2027. You have time, but the opt-out phase will make new Outlook the default for anyone you haven’t excluded by policy.
• Everyone: If you can’t sign in, check which account type you have before you start troubleshooting.