Outlook Password Prompts on Mac (Modern Authentication Window)
Section: Email & Microsoft 365 Tags: outlook, password-prompt, modern-authentication, mac, keychain, m365 Source tickets: #27413, #27106 Confidence: High — seen across multiple clients, escalated to Microsoft, root cause confirmedSymptom
Outlook on macOS shows a "Sign in to your account" (modern authentication) pop-up repeatedly:
- On launch
- Whenever Wi-Fi drops and reconnects
- When switching between networks (office ↔ home ↔ 4G)
- Entering the correct password does not dismiss it permanently; force-quitting Outlook and reopening temporarily fixes it
Root Cause
The Outlook credential token stored in macOS Keychain becomes stale or corrupt. This is a known Microsoft bug, worse on older Outlook builds (pre-16.30). The trigger is any network interruption that causes the OAuth token to be challenged — Outlook cannot silently re-authenticate and surfaces the prompt instead.
A secondary cause is a corrupt Exchange account profile file stored in ~/Library/Group Containers/.
Prerequisites
- Remote access to the affected Mac (TeamViewer / Screen Share)
- Outlook closed before running any steps
- Know which M365 account(s) the user has (primary + any shared mailboxes)
Remediation Steps
Step 1 — Update Outlook to the latest build
Open Outlook → Help → Check for Updates. If the machine is on an older build (below 16.40 as a rough guide), enable Insider Fast to get the newest token-handling fixes:
- In Microsoft AutoUpdate, tick "Join the Office Insider programme"
- Select Fast channel
- Install the update and reboot Outlook
Step 2 — Run NukeOffKeychain (recommended first-pass fix)
This Microsoft-provided tool removes all stale Outlook credential entries from Keychain without touching other data.
- Download:
https://github.com/pbowden-msft/NukeOffKeychain - Extract the zip to the Desktop
- Close Outlook completely (Cmd+Q, verify in Activity Monitor)
- Open Terminal and run:
chmod +x ~/Desktop/NukeOffKeychain-master/NukeOffKeychain
~/Desktop/NukeOffKeychain-master/NukeOffKeychain --Default --Force
- Relaunch Outlook — it will ask to sign in once. Enter credentials normally.
Step 3 — Delete the Exchange account profile file (if Step 2 fails)
The account binding file can become corrupt independently of Keychain.
- In Outlook: Outlook → Preferences → Accounts — note the account settings (server, username)
- Quit Outlook
- In Finder, press Cmd+Shift+G and go to:
~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/Exchange Accounts/
- You will see a file with no extension (one per Exchange account). Delete it.
- Relaunch Outlook — re-enter credentials when prompted.
Step 4 — Full Office uninstall + reinstall (if Steps 1–3 fail)
- Download and run the Microsoft Office Removal Tool from
https://support.microsoft.com/en-gb/office/uninstall-office-for-mac-eefa1199-5b58-43af-8a3d-b73dc1a8cae3 - Reboot
- Remove any remaining credentials: open Keychain Access, search for "microsoft" and "office", delete all entries
- Reinstall Office from
https://portal.office.com→ Install apps - Sign in with the user's M365 credentials
Step 5 — Enable verbose logging and raise Microsoft ticket (if all else fails)
This scenario may be account/mailbox-specific and require a Microsoft fix on the server side.
- In Outlook: Window → Sync Errors → Cog icon → Enable Logging for Troubleshooting
- Reproduce the prompt — capture the sync error log
- Raise a ticket at
https://admin.microsoft.com→ Support → New service request - Provide: Outlook build version, macOS version, the sync error log, and the list of affected mailboxes
Verification
After the fix, change Wi-Fi off and back on while Outlook is open. Outlook should reconnect silently without any password prompt appearing.
If This Fails
Check whether the issue is specific to shared mailboxes — if shared mailboxes are the only ones prompting, the issue is tenant-side OAuth delegation, not the client. Remove and re-add the shared mailboxes under File → Open & Export → Other User's Folder after clearing Keychain.
Known Affected Versions
Outlook 16.25 on macOS 10.14 (Mojave) was the most commonly reported combination in Purple Computing tickets. Issue largely resolved in builds 16.40+.
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