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Created January 2, 2026 11:53
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Mailbox migration to Office 365
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mailbox-migration.md

Mailbox Pre-Export Checklist

Preparation

  • Confirm user mailboxes to migrate
  • Ensure administrative access / delegated permissions
  • Verify Outlook Desktop installed and updated
  • Confirm stable network connection
  • Backup any existing local PST files

Mailbox Configuration

  • Enable Cached Exchange Mode
  • Set “Mail to keep offline” → All
  • Wait for full mailbox sync

Mailbox Cleanup

  • Move unread newsletters/notifications > 2 years → Archive
  • Move read receipts/confirmations > 1 year → Archive
  • Move old calendar notifications → Archive
  • Move old read mail > 3 years → Archive
  • Move large attachments > 5MB older than 2 years → Archive
  • Empty Deleted Items and Junk Email
  • Remove unnecessary mailbox rules

Verification

  • Compare item counts in Outlook vs OWA
  • Spot-check important emails, attachments, calendar events, and contacts
  • Ensure mailbox size is manageable for PST export

PST Export

  • Choose local folder with sufficient space
  • Export full mailbox including all folders
  • Verify folder structure and item counts in PST
  • Spot-check critical items in PST

OneDrive / SharePoint

  • Download important files from OneDrive / SharePoint (if any)

Other Considerations

  • Verify website contact form emails are forwarded correctly to mailbox
  • Document PST file location, date, and any notes on issues
  • Keep original mailbox intact until new tenant verified

Mailbox Post-Import Checklist

PST Import Verification

  • Import PST into the new tenant mailbox
  • Verify folder structure matches original mailbox
  • Check all emails are present (spot-check important messages)
  • Verify calendar items and recurring events
  • Verify contacts are complete and correct
  • Confirm flagged items and categories imported correctly
  • Confirm attachments open properly

Mailbox Configuration

  • Assign correct licenses and mailbox settings
  • Reapply mailbox rules (if any)
  • Enable Cached Exchange Mode with “Mail to keep offline” → All
  • Verify mailbox sync in Outlook and OWA

OneDrive / SharePoint

  • Upload any downloaded files to user OneDrive / SharePoint
  • Verify folder structure and permissions

Email Flow & Services

  • Test sending/receiving email from new tenant
  • Confirm website contact form emails are forwarding to new mailbox
  • Confirm any aliases or shared addresses are functional
  • Verify signature templates, automatic replies, and delegation (if applicable)

Final Verification

  • Have user review mailbox for missing items
  • Compare counts of Inbox, Sent Items, Drafts, Calendar, and Contacts with old PST
  • Confirm mobile devices or Outlook clients reconnect correctly
  • Document completion date, PST location, and notes on any issues
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