What has changed
Microsoft is gradually replacing classic Outlook for Windows with the “new Outlook”, a client built on the same code as Outlook on the web. It is rolling out in stages: first as an option people could try, then as the default with the ability to switch back. For enterprise customers, the new Outlook became the default experience from April 2026.
Many people have simply clicked the Try the new Outlook toggle without realising it changes how their mailbox connects. The new Outlook does not talk to Exchange the way classic Outlook does — it relies on Microsoft’s cloud services, which is why it supports Microsoft 365 mailboxes and not servers Microsoft does not run.
Which accounts the new Outlook supports
According to Microsoft’s own documentation, the new Outlook for Windows supports:
- Microsoft 365 work and school accounts (Exchange Online)
- Outlook.com and Hotmail
- Gmail, Yahoo and iCloud
- Other accounts that connect over IMAP or POP
Microsoft states plainly that on-premises Exchange accounts aren’t supported. Trying to add one produces the message “We couldn’t reach the email server. Please try again.” — which sends a lot of people looking for a server fault that does not exist.
Does this affect hosted Exchange?
Yes. From Outlook’s point of view, a hosted Exchange service running Exchange Server SE is an on-premises Exchange organisation — it is simply on someone else’s premises. That applies to us and to every other hosted Exchange provider. The only Exchange service the new Outlook supports is Microsoft’s own Exchange Online.
It is tempting to work around it by adding the mailbox to the new Outlook as an IMAP account. It will show your email, but you lose what made Exchange worth having: shared calendars, contacts sync, delegate access and the rest. We do not recommend it.
What works with Exchange Server today
- Classic Outlook for Windows — full support, including cached mode, shared mailboxes, delegate access and public folders. Microsoft has committed to supporting classic Outlook until at least 2029, and to giving at least 12 months’ notice before it is retired.
- Outlook on the web — the browser version of Outlook that comes with Exchange Server, on any operating system.
- iPhone, iPad and Android — the built-in mail and calendar apps connect over Exchange ActiveSync, with push email.
- Outlook for Mac — check the version: Microsoft has been rolling out on-premises Exchange support in its newest Mac client in stages, and the older “legacy” Mac Outlook continues to connect.
How to switch back to classic Outlook
If the new Outlook was switched on with the toggle, the same toggle in the top-right corner of the new Outlook switches it off again and reopens classic Outlook, with the original profile intact. If the new Outlook is the only Outlook on the PC, classic Outlook is installed as part of the Microsoft 365 Apps or Office installation and can be reinstated by whoever manages your PCs.
For an organisation, it is worth setting the default centrally rather than relying on each person to notice the toggle. Microsoft provides policies that control whether the new Outlook toggle appears and whether users are moved across automatically.
Public folders and the new Outlook
Public folders are a separate catch. Microsoft describes the new Outlook’s public folder support as limited, and advises anyone who depends on public folders to keep using classic Outlook. Even on Exchange Online, public folders have to be added to Favourites to appear, and creating or managing them still needs classic Outlook.
What happens after 2029?
Nobody outside Microsoft knows yet. Microsoft has said classic Outlook will be supported until at least 2029, with 12 months’ notice before any final cutover. Whether the new Outlook gains support for on-premises Exchange before then has not been announced.
For planning, that gives you a comfortable runway: classic Outlook is a fully supported client for years yet. If using the new Outlook specifically is important to your business, Exchange Online is currently the only Exchange service that supports it — and we will tell you that honestly if you ask, because we resell Microsoft 365 too.
How we handle it for customers
Every migration we run includes setting up classic Outlook profiles and making sure nobody has been quietly switched to the new Outlook. If a customer’s PC is moved across later by an update, a call to our support line puts it back in a couple of minutes.