Resolving iPhone Sign-In Failures for Outlook & Teams After Tenant Migration

Purpose of This Article

  1. What this article aims to show you:
    This guide explains how to resolve an issue where an iPhone refuses to sign into Outlook or Microsoft Teams after a Microsoft 365 tenant-to-tenant migration, even after reinstalling apps or resetting passwords.
  2. When to use this information:
    Use this process any time a migrated user’s iPhone cannot authenticate to Outlook/Teams, especially when:
    • Credentials are correct but apps repeatedly fail to sign in
    • The old tenant’s identity is still cached on the device
    • Standard app reinstalls do not fix the issue
      This problem occurs because iOS stores stubborn Microsoft account caches that persist across reinstalls.

Step-by-Step Resolution Procedure

1. Use Microsoft Edge’s Hidden Reset Function (Most Effective Step)

This clears deeply embedded Microsoft account tokens that Outlook/Teams cannot remove themselves.

  1. Install Microsoft Edge from the App Store (temporary use only).
  2. Open Edge and in the URL bar enter: edge://signin-internals
  3. Tap RemoveAllAccounts.
  4. A red confirmation pop-up will appear—this is expected.
  5. Close Edge completely.
  6. Reinstall Outlook and Teams after completing all steps below.

Why this matters:
Edge exposes internal MSAL (Microsoft Authentication Library) caches that other Office apps cannot purge. Clearing this nearly always resolves post-migration authentication issues on iOS.

2. Remove Old Work Accounts from iOS System Settings

Even if the old work account isn’t showing in Outlook, iOS may still be storing it at the system level.

  1. Go to Settings → Mail → Accounts and delete any old work accounts.
  2. Also check under:
    • Settings → Passwords & Accounts (older iOS versions)
    • Settings → Apple ID → iCloud → Safari → temporarily disable Safari sync
  3. Go to Settings → Safari → Clear History and Website Data.

This flushes stored autofill credentials and account references that cause the sign-in loop.

3. Remove Old Accounts From Any Office App

If the user has Word/Excel/OneDrive installed, they may still hold the old tenant account.

  1. Open any Office app (Word, Excel, OneDrive, etc.).
  2. Tap the user’s profile icon (top left).
  3. Go to Settings → Storage Accounts.
  4. Swipe left on any old or incorrect accounts → Delete.

This removes the identity from Microsoft’s app-wide shared credential store.

4. Fully Reinstall the Apps After Clearing Credentials

Once the caches and accounts have been removed:

  1. Delete Outlook and Teams.
  2. Restart the iPhone (important reset of system caches).
  3. Reinstall the apps fresh from the App Store.
  4. Sign in with new tenant credentials.

At this point, authentication should succeed without referencing the old tenant.

Recap / Final Notes

This issue happens frequently after Microsoft 365 tenant migrations because iOS retains hidden Microsoft identity caches that aren’t cleared during normal reinstall procedures. Using Edge’s internal account purge, combined with removing accounts from iOS settings and Office app storage, ensures all legacy tenant tokens are removed.

Once all caches are cleared and the apps reinstalled, the user should be able to log into Outlook and Teams normally using their new tenant credentials.

If this process fails (rare), escalate internally before re-engaging Microsoft support—this procedure resolves the majority of cases.