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Outlook Client Reconfiguration Process (Windows)

Below is the process for updating local Outlook client profiles on Windows after transitioning to a new platform. It is most commonly used after a Microsoft 365 tenant-to-tenant migration, and includes explicit cache-clearing steps required for your Outlook client to sync to your new target account.

When This Is Required

This process is required when:

  • Your mailbox was migated between Microsoft 365 tenants
  • The UPN, tenant ID, or mailbox GUID changes
  • Outlook continues trying to authenticate to the old tenant

Important: Simply re-entering credentials is usually insufficient. A new Outlook profile and cache cleanup are required.

High-Level Process (Summary)

  • Close Outlook completely
  • Remove cached credentials
  • Clear Outlook cache (OST + Autodiscover)
  • Create a new Outlook profile
  • Re-add the mailbox using new tenant credentials
  • Validate sync and functionality

Step-by-Step Instructions (Windows)

1. Close Outlook and Office Apps

  • Close Outlook
  • Ensure Outlook is not running:
    • Press Ctrl + Shift + Esc
    • End task for:
      • OUTLOOK.EXE
      • MSOSYNC.EXE (if present)

2. Remove Stored Credentials

  • Open Control Panel
  • Go to Credential Manager
  • Select Windows Credentials
  • Remove all entries related to:
    • MicrosoftOffice
    • Outlook
    • ADAL
    • MSOID
    • Old email address or tenant domain

3. Clear Outlook Cache Files (Critical)

  • Delete OST Files
    • Open File Explorer
    • Go to:
      • %LOCALAPPDATA%MicrosoftOutlook
    • Delete all .ost files
      • Note that they will be recreated automatically
  • Delete Autodiscover Cache
    • In File Explorer, go to:
      • %LOCALAPPDATA%MicrosoftOutlook
    • Delete:
      • *.xml files
      • *.tmp files
    • Optional, but recommended for stubborn cases:
      • %LOCALAPPDATA%MicrosoftOffice16.0OutlookProfiles
        • Delete only if Outlook fails after profile recreation

4. Create a New Outlook Profile

  • Open Control Panel
  • Go to Mail (Microsoft Outlook)
  • Click Show Profiles
  • Click Add
  • Name the profile (e.g., Outlook – New Tenant)
  • Enter:
    • New email address
    • New tenant password (or temporary password)
  • Allow Microsoft sign-in prompts (modern auth)

5. Set the New Profile as Default

  • In Show Profiles:
    • Select Always use this profile
    • Choose the new profile
  • Click OK

6. Open Outlook and Validate

  • When Outlook opens:
    • Confirm mailbox begins syncing to your target account
    • Expect initial sync delay for large mailboxes

7. Verify

  • Send/receive email
  • Folder structure
  • Calendar items
  • Contacts

Common Issues & Fixes

Outlook Keeps Prompting for Password

  • Credentials not fully removed
  • Repeat Credential Manager cleanup
  • Ensure no old tenant accounts remain signed in under:
    • https//portal.office.com
    • Sign out everywhere!

Mailbox Missing or Not Syncing

  • Confirm user is licensed in new tenant
  • Confirm mailbox exists in Exchange Online
  • Allow up to several hours for large OST rebuilds

If Looping To Your Old Tenant

Shared Mailboxes / Delegates Missing

  • Expected behavior
  • Shared mailboxes and permissions must be reassigned in the new tenant
  • Outlook automapping may take 30–60 minutes after reassignment

Best Practice (IT Recommendation)

  • For tenant-to-tenant migrations:
    • Always rebuild Outlook profiles
    • Do not attempt profile reuse
    • Clear cache before first sign-in
  • This prevents:
    • Authentication loops
    • Ghost mailboxes
    • Cross-tenant autodiscover failures