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

‍

The latest use cases

All our use cases

Modern Workplace Microsoft 365: Security, Compliance and Data Protection

As cyberthreats increase, strengthening Microsoft 365 security and compliance becomes essential. This use case shows how IT Systèmes helped an organization structure its Zero Trust posture and protect its sensitive data.

Merger and separation of Microsoft 365 tenants

Merging tenants, separating perimeters or migrating identities and data are now strategic operations, with high stakes in terms of security, service continuity and governance. This use case illustrates how IT Systèmes supported a customer in the complete merger of two M365 tenants.

Optimize Microsoft 365 usage with Copilot M365 & Copilot Studio

This use case illustrates how IT Systèmes helped a customer set up a structured, secure and governed approach based on Copilot.