Exchange OnPremise to Office 365 migration - Deployment and automation in a modern infrastructure

Migration to Exchange Online is part of a strategy to modernize messaging, reduce infrastructure costs and improve security. It involves a profound transformation of flows, identities and governance processes.

CIO objectives when migrating from Exchange to Office 365

  • Service continuity and minimized downtime
  • Data security and compliance (RGPD, PRA/PCA)
  • Streamlined administration and automation

Technical prerequisites for a successful migration

  • Inventory and mapping of the Exchange environment (versions, databases, DAGs, connectors, namespaces, certificates)
  • Directory synchronization: implementation of Azure AD Connect (hybrid mode, OR filtering, attribute management)
  • DNS preparation: domain validation, configuration of Autodiscover, MX, SPF, DKIM, DMARC records
  • Security: TLS encryption, MFA, Conditional Access, external access management
  • Licenses: allocation of Office 365 licenses (Exchange Online Plan 1/2, etc.)

Exchange OnPremise to Office 365 migration architecture

  • Hybrid mode (Hybrid Modern Auth recommended): allows coexistence, gradual migration, centralized management of mailboxes and SMTP flows
  • Exchange Hybrid Configuration Wizard (HCW): configuration of mail flows, delegation, free/busy, batch migration, etc.
  • Object management: synchronization of shared boxes, resources, distribution groups, delegated permissions

Our approach to migrating from Exchange OnPremise to Office 365

1. Preparing the environment

  • Exchange v20xx update (latest CU, security patches)
  • AD health check, replication, obsolete object cleanup
  • Virtual URL configuration (OWA, EWS, MAPI, Autodiscover)
  • Deploying Azure AD Connect (with Password Hash Sync or Pass-Through Auth)

2. Hybrid mode configuration

  • Running HCW (Hybrid Configuration Wizard)
  • Publication of necessary services (EWS, Autodiscover) via reverse proxy or firewall
  • Configuration of mail flow connectors (inbound/outbound) between Exchange Online and On-Prem
  • Enable identity federation (if SSO required)

3. Mailbox migration

  • Planning of migration batches (by department, site, criticality)
  • Migration endpoints (MRS Proxy)
  • Migration monitoring (logs, alerts, PowerShell reporting)
  • Archive management, shared boxes, delegates, access rights

4. Post-migration and optimization

  • Flow validation (mailflow, calendars, mobile access)
  • Object cleaning, deactivation of empty bases
  • Update administration scripts, documentation, runbooks
  • Training of support teams and user communication

Security and compliance after Exchange Online migration

  • MFA/Conditional Access: enhanced access to Exchange Online boxes
  • Audit and traceability: mailbox audit activation, DLP alerts, log supervision
  • Advanced protection: activate Exchange Online Protection (EOP), Defender for Office 365 (ATP)
  • RGPD compliance: managing the right to be forgotten, retention, eDiscovery

Office 365 migration results

  • Reduced attack surface (end of legacy on-prem, simplified patching)
  • Flexibility and scalability (adding boxes, mobility, global access)
  • Automation (PowerShell, Graph API, advanced reporting)
  • Improved resilience (Microsoft SLA, native PRA/PCA)

Points to watch when migrating from Exchange OnPremise to Office 365

  • Management of third-party applications using Exchange (fax, copiers, business applications)
  • Migration of local archives (PST, third-party solutions)
  • Governance of access rights and delegations
  • Communication and change management

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