Assumptions:

1. Hybrid Exchange Migration and Stage Migration methods have been considered and rejected
2. Active Directory Sync has been deployed and on premise users and groups have been synchronized onto Office 365

Procedures:
Overview
Export PST of the user > Disable the mailbox within on premise Exchange > Trigger manual Azure AD Sync > Upload the mailbox onto O365 > Wait until mailbox object was updated on O365 without a mailbox feature > Add or reset the user account to trigger creation of new mailbox feature > Import PST to user account’s mailbox
 
Technical instructions: https://learn.microsoft.com/en-us/purview/use-network-upload-to-import-pst-files
Preparation Phase

1. Add all LTD domains onto Office 365
2. Enable relay from local Public IPs to Office 365
4. Create multiple csv files with information on mailbox exporting batches
5. Export on premise Exchange mailboxes to local storage as PST files
6. Install Azure Az Copy
7. Sign up for Azure Storage
8. Add a Global Admin account into “Import Export Management” group, and then add “Import Export Management” into “Organizational Management” group

Pilot Phase

1. Send “batch 1” to Azure Storage
2. Prepare csv files with information on mailbox importing batches
3. Import “batch 1” to Office 365
4. Validate importing success of batch 1
5. Test from inside network > Manually configure email clients (Outlook) to use Office 365 URLs on batch 1 > Confirm successful mail flows (sending and receiving) of email clients of batch 1 accounts
6. Test from outside the network > Manually configure email clients (Outlook) to use Office 365 URLs on batch 1 > Confirm successful mail flows (sending and receiving) of email clients of batch 1 accounts

Roll-back Plan of Pilot Phase
1. Manually reconfigure email clients to use on premise Exchange
2. Confirm successful mail flows of test accounts

Cut-over Phase

1. Make storage & bandwidth calculations
2. Write detailed “run book” on cutover procedure, including Roll-back Plan
3. Practice following the run book to detect potential technical difficulties and time duration feasibility. Make adjustments to run book if necessary.
4. Schedule cutover date and mail outtage time window
5. Obtain approval from Stake Holders that external email access will be unavailable during scheduled cutover
6. Configure internal and external DNS on all top level domains to point to Office 365: MX, SPF, autodiscover, smtp/mail
7. Send “batch 2” to Azure Storage
8. Import batch 2
9. Validate importing success of batch 2
10. Test from inside network > Restart email clients > Confirm successful mail flows (sending and receiving) of email clients of batch 2 accounts
11. Test from outside the network > Restart email clients > Confirm successful mail flows (sending and receiving) of email clients of batch 2 accounts

Roll-back Plan of Cutover Phase
1. Reverse DNS changes
2. Restart all email clients and validate automatic reconnections from inside the network
3. Validate reconnections of email clients from outside the network

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