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Outlook Client Reconfiguration Process (Mac)

Below is the process for updating local Outlook client profiles on a Mac OS 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 migrated 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 (Mac)

1. Close Outlook

  • Quit Outlook
  • Confirm it is not running via Activity Monitor

2. Remove Outlook Profile

  • Open Outlook
  • Go to Outlook > Settings > Accounts
  • Select the old account
  • Click – (Remove)

3. Clear Outlook Cache (Mac)

  • In Finder:
    • Click Go → Go to Folder
    • Navigate to:
      • ~/Library/Group Containers/UBF8T346G9.Office/Outlook
    • Delete contents of:
      • Outlook 15 Profiles
      • Data
      • Cache
    • Important! Do not delete the entire Office folder — only delete Outlook-related folders.

4. Re-add Account

  • Open Outlook
  • Add account using new Microsoft 365 credentials
  • Complete Microsoft sign-in flow

Common Issues & Fixes

Outlook Keeps Prompting for Password

  • Credentials not fully removed
  • 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