Every few months another wave of "classic Outlook is dying" headlines makes the rounds, and every few months the actual dates move. Here's what Microsoft's own documentation says as of August 2026, and the policies that let you control the transition instead of being dragged through it.

The Actual Timeline

Microsoft runs the migration in three stages, documented on its product availability roadmap.

Opt-in, happening now. New Outlook is off by default. Users see a "Try the new Outlook" toggle and can switch back freely. Both versions run side by side.

Opt-out, next. New Outlook becomes the default, but users can still revert. Microsoft commits to at least 12 months of notice before this stage hits managed enterprise plans.

Cutover, last. No switching back. Also preceded by at least 12 months of notice for production rings.

The date that matters for planning: Microsoft states that existing classic Outlook installations, both perpetual and subscription, remain supported until at least 2029. Nobody's classic Outlook is being deleted this year. The realistic risk isn't losing the app, it's waking up one stage deeper than you planned for because the notices went to an unwatched admin inbox.

Microsoft: stages of migration to new Outlook

Control It: Keep Users on Classic

If your line-of-business add-ins, shared mailbox workflows, or PST archives aren't ready, stop the migration prompts with the migration policy set to disabled. The registry equivalent, per user:

reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v DoNewOutlookAutoMigration /t REG_DWORD /d 0 /f

The same setting exists as the "Admin-Controlled Migration to New Outlook" policy in Group Policy, Cloud Policy, and Intune administrative templates if you manage at scale.

Control It: Move Users to New Outlook

Going the other way, the same policy set to 1 walks users through a staged migration: a suggestion first, a warning next session, then an automatic switch. Pair it with the retry interval policy to re-prompt users who toggle back:

reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v DoNewOutlookAutoMigration /t REG_DWORD /d 1 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v NewOutlookAutoMigrationRetryIntervals /t REG_DWORD /d 30 /f

To lock migrated users in, hide the switch-back toggle with Exchange Online PowerShell:

Set-OwaMailboxPolicy -Identity OwaMailboxPolicy-Default -HideClassicOutlookToggleOut $true

Microsoft: admin-controlled migration policy

What to Do Before 2029

Inventory what actually blocks you: COM add-ins, PST-heavy workflows, and offline-first users are the common ones. New Outlook is web-based under the hood and handles these differently or not at all. Test the new client against your real workflows during the opt-in stage while switching back is still free, and decide your migration date instead of letting the notice cycle decide it for you.

Outlook Shared Calendar Crashes and Sync Bugs: Fix

Fix Classic Outlook MSVCP140.dll Crashes

Something Went Wrong in Outlook, Teams, and Microsoft 365

Planning an Outlook migration without breaking your add-ins and shared mailboxes? Contact Rain City Techworks.