Overview & M&A Business Context

Corporate Mergers, Acquisitions, and Divestitures (M&A) require rapid IT consolidation. Operating multiple isolated Microsoft 365 Tenants leads to fragmented collaboration, duplicate licensing costs, compliance liabilities, and poor brand unity.

Executing a Microsoft 365 Tenant-to-Tenant (T2T) Migration requires synchronizing core workloads:

1. Identities & Entra ID User Objects
2. Exchange Online Mailboxes & Archives
3. OneDrive for Business & Personal Files
4. SharePoint Online Sites & Document Libraries
5. Microsoft Teams Channels, Chat History & Files
6. Custom Vanity Domains (e.g., @company.com)

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Phase 1: Entra ID Cross-Tenant Trust & Service Principal Setup

Native Cross-Tenant Mailbox Migration relies on Entra ID Cross-Tenant Access Policies and dedicated Service Principals in both Source and Target tenants.

1. Configure Organization Relationships

Execute on both Source and Target Exchange Online endpoints:

# Connect to Exchange Online Source Tenant

Connect-ExchangeOnline -UserPrincipalName admin@sourcetenant.com



# Enable Cross-Tenant Mailbox Move Endpoint

New-MigrationEndpoint -AccessMethod CrossTenant -Name "T2T_Target_Endpoint" `

    -AppId "e0d0c0b0-a090-8070-6050-403020100000" `

    -ServiceEndpoint "https://outlook.office365.com/EWS/Exchange.asmx" `

    -TargetDeliveryDomain "targettenant.mail.onmicrosoft.com"

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Phase 2: Native Cross-Tenant Mailbox & OneDrive Migration

Unlike legacy EWS scraping, native M365 Cross-Tenant migration moves mailboxes directly at the storage fabric layer using Mailbox Replication Service (MRS).

Benefits of Native MRS Cross-Tenant Moves:

• Speeds up to 80GB/hour per mailbox stream
• Preserves Outlook OST files (no complete client re-download needed)
• Moves Archives and Hold Data intact

# Initiate Cross-Tenant Request for User

New-MoveRequest -Identity "user@sourcetenant.com" `

    -TargetDeliveryDomain "targettenant.mail.onmicrosoft.com" `

    -RemoteCredential (Get-Credential) `

    -RemoteHostName "outlook.office365.com" `

    -Flags Parallel

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Phase 3: Vanity Domain Cutover Execution Protocol

The most critical step during a weekend migration is releasing the primary custom domain (@epifive.com) from Source Tenant A and binding it to Target Tenant B.

Automated Domain Scrubbing Script

# Connect to Source Tenant

Connect-MgGraph -Scopes "User.ReadWrite.All", "Domain.ReadWrite.All"



$DomainToMove = "epifive.com"

$FallbackDomain = "sourcetenant.onmicrosoft.com"



# 1. Strip Domain from UserPrincipalNames

$Users = Get-MgUser -All | Where-Object { $_.UserPrincipalName -like "*@$DomainToMove" }

foreach ($u in $Users) {

    $NewUPN = $u.UserPrincipalName.Replace("@$DomainToMove", "@$FallbackDomain")

    Update-MgUser -UserId $u.Id -UserPrincipalName $NewUPN

    Write-Host "Updated UPN for $($u.DisplayName) -> $NewUPN" -ForegroundColor Green

}



# 2. Strip Domain from ProxyAddresses

$AllUsers = Get-MgUser -All

foreach ($u in $AllUsers) {

    $CleanProxies = $u.ProxyAddresses | Where-Object { $_ -notlike "*@$DomainToMove" }

    Update-MgUser -UserId $u.Id -ProxyAddresses $CleanProxies

}



# 3. Remove Domain from Source Tenant

Remove-MgDomain -DomainId $DomainToMove -Force

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Phase 4: Microsoft Teams Cross-Tenant Migration Strategy

Microsoft Teams structure consists of Chat History, Files (stored in SharePoint/OneDrive), and Apps.

1. Team & Channel Schema Provisioning: Re-create Team names, channels, private channel memberships, and tab configurations in Target Tenant B via Microsoft Graph API.
2. Channel Chat History HTML Injection: Export source channel messages and inject them as historical HTML transcript posts or native Graph Chat messages into destination channels.
3. File Migration: Move underlying SharePoint site assets linked to Teams channels using Microsoft Migration Manager.

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Post-Migration Verification & Health Checklist

• [x] All primary MX records verified with EOP SPF, DKIM, and DMARC enforcement (v=spf1 include:spf.protection.outlook.com -all).
• [x] Auto-redirects enabled on former OneDrive URLs.
• [x] Entra ID Conditional Access policies applied to newly ingested users in Target Tenant.
• [x] License assignment re-allocated via Entra ID Group-Based Licensing.