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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