Exchange Server to Exchange Online Migration Guide 2026

Microsoft ended mainstream support for Exchange Server 2016 in 2025, and Exchange Server 2019 support is winding down. If you're still running on-premises Exchange, now is the time to plan your move to Exchange Online.

This guide covers the main migration paths, key decisions, and best practices based on our experience migrating hundreds of organizations.

Migration Path Options

1. Cutover Migration (Best for Small Organizations)

• All mailboxes moved in a single weekend or short window
• Simple DNS changes (MX, Autodiscover, etc.)
• Best for: < 150–200 mailboxes, low complexity, minimal public folders
• Downsides: Limited coexistence, higher risk if issues arise

2. Staged Migration (Good for Medium Environments)

• Move users in batches over days or weeks
• Requires Azure AD Connect / Entra Connect for identity sync
• Best for: 200–2,000 mailboxes with some need for gradual transition

3. Hybrid Migration (Recommended for Most Organizations > 150 Users)

• Maintains coexistence between on-premises and Exchange Online
• Uses Hybrid Configuration Wizard
• Supports mailbox moves with minimal downtime
• Best for: Larger organizations, those wanting long-term hybrid, or complex requirements (public folders, unified messaging, etc.)

Pre-Migration Checklist

Assess Your Current Environment

• Inventory all mailboxes, sizes, and archive status
• Document public folders (many organizations still have significant data here)
• Review mail flow rules, transport rules, and journaling
• Check for legacy devices and applications using MAPI, POP, or IMAP
• Validate Active Directory health (critical for hybrid)

Licensing & Prerequisites

• Ensure you have appropriate Microsoft 365 licenses (Exchange Online Plan 1 or 2, or E3/E5)
• Set up Entra Connect (Azure AD Connect) if doing hybrid or staged
• Configure accepted domains and email address policies in the target tenant

Public Folders

Public folder migration is one of the most complex parts of Exchange migrations. Decide early:

• Migrate to modern public folders in Exchange Online
• Move content to SharePoint/OneDrive instead
• Archive and decommission

Execution Best Practices

Hybrid Configuration

Run the Hybrid Configuration Wizard to establish secure mail flow and free/busy sharing between environments. This is essential for a smooth experience.

Mailbox Moves

• Use batch moves with the Mailbox Replication Service (MRS)
• Monitor move reports for errors (bad items, large items, permissions issues)
• Perform test moves with pilot users first
• Schedule large moves during off-hours

Mail Flow & DNS

Plan your cutover carefully:

• Update MX records to point to Microsoft 365
• Configure Autodiscover
• Set up connectors for any on-premises applications or printers that send email
• Test inbound and outbound mail thoroughly

Client Configuration

Most modern Outlook clients will reconfigure automatically via Autodiscover. However, plan for:

• Older Outlook versions (2016 and earlier may need updates)
• Mobile devices (iOS/Android)
• Conference room systems and shared mailboxes

Common Challenges & Solutions

| Challenge | Solution | |-----------------------------|----------| | Large mailboxes or archives | Use archive mailboxes or split migrations | | Public folders | Dedicated public folder migration project or move to SharePoint | | Legacy applications | Identify and update or replace before cutover | | Permissions & delegates | Validate after moves; use PowerShell to audit | | Free/busy issues | Hybrid configuration and proper Autodiscover setup |

Post-Migration Steps

1. Decommission on-premises Exchange servers (after full validation period)
2. Clean up mail-enabled groups and contacts
3. Implement Microsoft 365 security features (Defender for Office 365, DLP, Conditional Access)
4. Train users on the new Outlook on the web and desktop experience
5. Set up retention policies and eDiscovery as needed

Is Hybrid Right for You Long-Term?

Many organizations start with hybrid for migration and then fully decommission on-premises Exchange. Others keep a lightweight hybrid presence for specific needs (certain compliance requirements or legacy apps).

Our team can help you decide the right long-term architecture.

Ready to Move Off Exchange Server?

Whether you're planning a cutover this quarter or a phased hybrid migration over the next year, Accred Consulting can help.

Schedule a free Exchange migration assessment: Contact us

We'll review your current setup, recommend the optimal path, and deliver a clear project plan with fixed pricing.

Frequently Asked Questions

How much downtime will users experience? With hybrid or well-planned staged migrations, most users see little to no downtime. Cutover migrations typically involve a weekend window.

What about public folders? We handle public folder migrations as a distinct workstream. Many clients choose to modernize content into SharePoint during the project.

Can we keep some mailboxes on-premises? Yes, but we generally recommend moving everything possible to Exchange Online for simplified management and better security.