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 confirmed

Symptom

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

Shared mailboxes can trigger one pop-up per shared mailbox on every launch.

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

Expected result: Many clients see the prompts stop entirely after a build update alone. Test before proceeding. Risk: None. Insider Fast builds are production-stable for M365 business.

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.

Expected result: Single sign-in prompt on relaunch, then no further prompts. Risk: None — only removes Outlook-specific Keychain entries. All other app credentials untouched.

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.

Expected result: Fresh account binding, prompts resolved. Risk: Caution — user will need to re-enter password. Signatures and rules are unaffected; they live server-side.

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

Expected result: Clean credential store. Prompts resolved. Risk: None to data; all email is server-side. Takes ~20 minutes.

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

Note: If multiple users on the same tenant are affected, this confirms a server-side issue and Microsoft must resolve it. There is nothing further to do on the client.

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+.