Complete Guide to Migrating from Exchange 2025 to Office 365 with a Hybrid Approach

Migrating from an on-premises Exchange environment, such as Exchange 2025, to a cloud solution like Microsoft 365 can significantly enhance your organization’s collaboration capabilities, flexibility, and overall IT efficiency. This guide covers the migration process in four parts and begins by explaining the core concepts and benefits of hybrid migration. By understanding the hybrid model, you can plan a smoother, more effective migration that supports both on-premises and cloud mailboxes during the transition.

What Is a Hybrid Migration?

A hybrid migration creates a bridge between your existing on-premises Exchange environment and the cloud solution. This allows for the migration of mailboxes while maintaining seamless coexistence between the two environments. The hybrid model is designed to enable smooth migration and collaboration across both platforms. Unlike a cutover migration, where all mailboxes are moved at once, or a staged migration that moves mailboxes in batches, hybrid migration allows you to synchronize mail flow, free/busy calendar data, and user accounts in real-time across both environments.

This type of migration is ideal for medium to large organizations, especially those with more than 150 mailboxes, as it provides the flexibility to move users gradually without disrupting daily operations. Hybrid migrations are beneficial for organizations that need to synchronize their directories, support directory synchronization, or require a phased migration approach.

Core Components of the Hybrid Migration Model

A hybrid deployment integrates several key services and tools to ensure seamless coexistence between the on-premises Exchange environment and Microsoft 365. The most important components involved in hybrid migration include:

Hybrid Configuration Wizard (HCW)

The Hybrid Configuration Wizard is a tool provided by Microsoft to help configure and establish the hybrid connection between the on-premises Exchange server and Microsoft 365. This wizard automates many of the key tasks involved in setting up hybrid configurations, such as setting up mail routing, free/busy sharing, and directory synchronization. By running the HCW, you will ensure that mail flow and calendar sharing work correctly between environments.

Azure Active Directory Connect

Azure AD Connect is a critical tool that synchronizes user accounts and credentials from on-premises Active Directory to the cloud-based directory service. This tool facilitates single sign-on (SSO) and enables users to maintain consistent login credentials across both Exchange environments. Once directory synchronization is established, users can easily transition to the cloud while keeping their identities and authentication settings intact.

Federation Trust and Organization Relationships

In a hybrid deployment, federation plays a central role in enabling secure communication between on-premises and cloud environments. Federation creates a trust relationship that allows users from different systems to collaborate. The federation trust is established with the Microsoft Federation Gateway, while organization relationships are set up with Microsoft 365 to enable shared calendar access and other coexistence features.

Mail Flow Configuration

One of the key aspects of hybrid migration is configuring mail flow. Hybrid deployments allow for different mail flow options, such as centralized or cloud-only mail transport. Centralized mail transport forces all outbound email to be routed through the on-premises Exchange server, whereas cloud-only mail transport directs messages from Exchange Online to external recipients. The choice of mail flow configuration depends on the organization’s needs and policies.

Autodiscover and Free/Busy Access

Autodiscover is a service that helps direct email clients to the appropriate mailbox location, whether on-premises or in the cloud. In a hybrid environment, the Autodiscover service ensures that users’ email clients automatically connect to the correct mail server. Free/busy access allows users to see calendar availability for other users, regardless of where their mailbox resides. Both features are crucial for maintaining a seamless experience during migration.

Benefits of the Hybrid Migration Model

The hybrid migration model offers a variety of advantages that make it a preferred choice for many organizations:

Phased Migration Approach

Hybrid migration enables you to move users gradually over time, reducing the impact on day-to-day operations. This phased approach also allows IT teams to test and validate configurations, ensuring that everything works as expected before completing the full migration. Migrating mailboxes in smaller batches also makes it easier to identify and resolve issues early on, improving the overall migration experience.

Seamless End-User Experience

Because the hybrid migration maintains coexistence between on-premises and cloud mailboxes, users experience little to no disruption during the transition. They can continue to send emails, schedule meetings, and access shared resources without needing to worry about where their mailbox is located. This seamless experience enhances user productivity and helps reduce the burden on IT support.

Retaining On-Premises Infrastructure

Hybrid migration allows you to retain your on-premises Exchange infrastructure, which can be particularly useful for organizations with specific compliance or technical requirements. Some organizations may need to keep certain mailboxes on-premises for regulatory or legal reasons. Hybrid migration allows these mailboxes to coexist with cloud mailboxes while maintaining the necessary infrastructure.

Secure Mail Flow and Directory Synchronization

The hybrid model ensures that mail flow remains secure and reliable, both internally and externally. By using directory synchronization tools like Azure AD Connect, organizations can maintain consistent user credentials across both systems. Additionally, centralized control over mail delivery and authentication makes it easier to enforce security policies and manage mail routing effectively.

Supports Large Organizations

For large organizations with thousands of users, hybrid migration provides the scalability and automation necessary to migrate mailboxes in manageable batches. The use of automated tools and scripts streamlines the migration process, reducing manual effort and minimizing the risk of errors.

Limitations and Considerations

While hybrid migration offers numerous benefits, it also comes with some challenges and limitations that should be carefully considered before proceeding:

Complexity and Technical Skill

Setting up a hybrid migration requires a solid understanding of both Exchange Server and Microsoft 365. Misconfigurations or a lack of experience can lead to synchronization issues, mail delivery problems, or unexpected disruptions. Organizations should ensure that their IT staff is adequately trained and familiar with the tools and processes involved in a hybrid deployment.

Infrastructure Requirements

A hybrid migration requires that the on-premises Exchange environment remains operational for the duration of the migration. This means maintaining hardware, SSL certificates, network bandwidth, and IT staff with expertise in both Exchange and Microsoft 365. Additionally, you will need to ensure that your network and firewall settings are configured to allow secure communication between on-premises servers and the cloud.

Firewall and Connectivity

For a successful hybrid migration, secure communication must be established between your on-premises Exchange Server and Microsoft 365. This typically involves opening specific ports on your firewall and ensuring that your Exchange server is externally accessible with a trusted SSL certificate. Without proper connectivity and security measures in place, mail flow and directory synchronization can be disrupted.

Licensing Considerations

While Microsoft 365 licenses are included in the migration to Microsoft 365, organizations must continue to maintain their on-premises Exchange licenses until the migration is complete and the on-premises infrastructure is decommissioned. It’s essential to account for the costs associated with maintaining both environments during the hybrid transition.

When Should You Use a Hybrid Migration?

Hybrid migration is the best option under the following circumstances:

• Your organization needs to migrate users gradually over time, rather than all at once.

• You have more than 2,000 mailboxes, making cutover or staged migrations impractical.

• You require specific coexistence features, such as calendar sharing and global address list synchronization, between on-premises and cloud users.

• Some mailboxes must remain on-premises due to regulatory compliance, legacy applications, or other technical reasons.

• You need centralized control over identity management and mail flow, rather than managing everything in the cloud.

Hybrid Migration Architecture Overview

Understanding how the hybrid environment is structured is essential for planning and implementing the migration. The hybrid architecture includes several critical components, such as directory synchronization, mail flow, and coexistence features.

Directory Synchronization

Azure AD Connect ensures that user accounts and passwords are synchronized between your on-premises Active Directory and Microsoft 365’s Azure Active Directory. This synchronization allows users to access cloud resources using the same credentials they use on-premises.

Mail Flow

Mail flow in a hybrid migration can be configured to meet your organization’s needs. Depending on your preferences, you can use centralized mail transport, cloud-only mail flow, or a split configuration that uses both systems for routing messages.

Coexistence Features

Hybrid environments support several features that enable seamless collaboration between users on Exchange Server and Microsoft 365. These features include shared calendar availability, unified global address lists, and the ability to track messages across both systems.

Client Access

Users can access their mailboxes using Outlook clients or web-based interfaces, with Autodiscover automatically directing them to the appropriate server. The hybrid migration model ensures that users do not need to reconfigure their clients, making the transition easier.

We walked through the key steps of setting up the hybrid environment and syncing directories. Now that the hybrid configuration is complete and your users are synchronized with Microsoft 365, it’s time to begin migrating mailbox data. This phase includes migrating mailbox content, testing the migration, and resolving any issues that arise along the way to ensure a smooth transition for end-users.

In this part of the guide, we will focus on how to handle the actual mailbox migration, troubleshoot common issues, and ensure a seamless transition to Microsoft 365 for all users.

Reviewing Your Environment Before Migration

Before migrating any mailbox data, it is important to perform a thorough review of your existing Exchange Server environment to ensure everything is ready for the migration process. This step helps avoid complications and ensures that the migration proceeds smoothly.

Checking Mailbox Sizes

One of the first tasks to consider is checking the size of your mailboxes. Larger mailboxes can take more time to migrate, and in some cases, they may require additional bandwidth or resources to complete the migration successfully.

• Review the mailbox size limits for Microsoft 365 and compare them to the largest mailboxes in your organization.

• If any mailboxes exceed the Microsoft 365 limits, consider archiving or reducing the size of these mailboxes before the migration. This can help prevent delays and migration errors.

Reviewing Mailbox Permissions

In many organizations, users may have specific permissions assigned to their mailboxes, such as full access, send-as, or send-on-behalf permissions. It’s important to document these permissions for each mailbox before migrating, as they will need to be recreated in Microsoft 365 after the migration if they aren’t automatically carried over.

• Document the permissions assigned to each mailbox.

• Check for any special delegation permissions or shared mailboxes that may need to be manually configured in Microsoft 365 after migration.

Preparing End Users

Informing and preparing end-users is a critical step in the migration process. Users should understand the migration schedule, what to expect before, during, and after the migration, and any potential service disruptions.

• Send out communications to inform users about the planned migration dates.

• Provide guidelines on what to expect during the migration, including any downtime or changes in how they access their email.

• Offer support resources to help users troubleshoot issues that may arise during the migration.

Performing Test Mailbox Migrations

Before migrating the entire organization, it’s highly recommended to conduct a pilot migration with a small group of users. Testing the migration process with a limited number of mailboxes helps identify potential issues before they affect all users.

Selecting Test Mailboxes

Choose a representative mix of users from different departments or those who have different types of mailbox usage. This ensures that a wide range of scenarios, including various mailbox sizes and permission setups, are tested.

• Select users from different departments or with varying mailbox sizes.

• Ensure the pilot group includes users with different levels of permissions, such as full access, shared mailboxes, and delegation permissions.

Initiating the Migration

To begin the test migration:

1. Navigate to the Exchange Admin Center (EAC) in Microsoft 365.

2. Go to Recipients > Migration and click + > Migrate to Exchange Online.

3. Choose the appropriate migration type, such as Cutover Migration, for the Express Hybrid configuration.

4. Select the mailboxes of the test users.

5. Complete the migration batch wizard.

Once the migration batch is created, monitor the progress of the migration to ensure there are no errors.

Monitoring Performance and Errors

As the test migration progresses, closely monitor the status of the mailbox moves. Look for any issues, such as network timeouts, corrupted items, or permission conflicts, that could cause delays or failures. Use the Exchange Admin Center and PowerShell scripts to gather detailed information about the migration status and troubleshoot any problems.

• Monitor the migration logs for errors related to mailbox sizes, permissions, or network issues.

• If necessary, troubleshoot issues using the error messages provided by Microsoft 365 or consult the Microsoft 365 support documentation for resolution steps.

Troubleshooting Common Migration Issues

Even well-planned migrations can encounter issues. Here are some of the most common problems you may face during mailbox migration, along with tips for resolving them.

Stalled or Slow Migrations

Slow or stalled migrations can occur due to various factors, such as large mailbox sizes, insufficient bandwidth, or throttling by Microsoft 365.

• Schedule mailbox moves during off-peak hours to reduce the impact on network traffic.

• Migrate large mailboxes in smaller batches to prevent network congestion.

• If throttling is causing delays, contact Microsoft support to request a temporary increase in migration throttling limits.

Mailbox Item Limits

During migration, some items may be skipped if they exceed the limits supported by Microsoft 365, such as attachments or calendar entries.

• Review the migration logs for details about skipped items, such as emails with large attachments or calendar events with non-standard formats.

• Advise users to clean up their mailboxes by archiving old emails or deleting unnecessary attachments before migration.

Mail Flow Interruptions

Mail flow issues may arise if mail routing or connectors are misconfigured between the on-premises Exchange Server and Microsoft 365.

• Double-check your send and receive connectors in both environments to ensure they are correctly set up.

• Test mail flow using Microsoft’s Remote Connectivity Analyzer to confirm that messages are being routed correctly between the cloud and on-premises environments.

• Review and verify accepted domains in Microsoft 365 to ensure they match the domains used by your on-premises Exchange Server.

Calendar and Free/Busy Sharing Issues

Calendar sharing and free/busy information can sometimes fail to synchronize between on-premises and cloud mailboxes during migration.

• Verify that organization relationships and federation trusts are correctly configured.

• Test calendar and free/busy access between users in the cloud and on-premises environments to ensure proper synchronization.

Migrating All User Mailboxes

Once the test migration is successful and any issues have been resolved, it’s time to proceed with migrating all user mailboxes.

Planning Your Migration Batches

When migrating a large number of users, it’s important to break the migration into smaller batches. This will prevent system overload and make it easier to monitor progress. Migrating in batches also allows you to prioritize critical users or departments and address any issues before migrating the entire organization.

• Prioritize users who are most critical to the organization’s operations.

• Avoid migrating all users on the same day to prevent overwhelming the migration process.

• Allow buffer time between batches to resolve any unexpected issues.

Automating Migration with PowerShell

To streamline the migration process, you can use PowerShell to automate and bulk manage the migration tasks. PowerShell scripts can help with tasks such as starting migration batches, assigning licenses, checking migration statuses, and reporting on errors.

• Use PowerShell to initiate and monitor migration batches for large numbers of users.

• Automate license assignments to reduce the manual effort involved in managing user accounts.

Verifying Mailbox Functionality Post-Migration

After completing the mailbox migration, it’s essential to verify that all mailbox content has been successfully transferred and that users can access their new Microsoft 365 mailboxes without issues.

Testing Access

Users should be able to access their email, calendar, contacts, and tasks after the migration. Perform testing by having users log in to their Microsoft 365 accounts using both the web interface (Outlook Web App) and desktop Outlook clients.

• Ensure that users can send and receive email without issues.

• Verify that calendar events, contacts, and tasks are accessible in the new environment.

• Test the search functionality within the mailbox to ensure that it is working correctly.

Verifying Delegated Access

If users had access to shared mailboxes or delegate permissions before the migration, verify that these permissions are still intact after the migration. In some cases, you may need to manually restore these permissions in Microsoft 365.

• Confirm that users can access shared mailboxes and calendars.

• Reassign any delegated access permissions that were not transferred during the migration.

Checking Mobile Device Synchronization

Many users access their email through mobile devices. Ensure that users can successfully connect to Microsoft 365 on their smartphones and tablets.

• Verify that mobile clients are updated with the correct account settings for Microsoft 365.

• Instruct users to remove and re-add their email accounts if needed to ensure proper synchronization.

Ensuring Continued Synchronization and Coexistence

During the hybrid migration, on-premises and cloud mailboxes will coexist for some time. It’s essential to ensure proper synchronization and mail flow between both environments until the migration is fully complete.

Directory Synchronization

Monitor the synchronization status between your on-premises Active Directory and Azure AD to ensure that no sync errors occur. Regularly check the synchronization logs to confirm that all user accounts are updated correctly.

• Use Azure AD Connect to ensure continued synchronization between your on-premises and cloud directories.

• Address any synchronization errors promptly to prevent delays in the migration process.

Hybrid Mail Flow

Ensure that hybrid mail flow continues to work correctly, allowing emails to be routed between on-premises and cloud mailboxes. Check your send connectors and mail routing configurations to confirm that email delivery is working as expected.

Unified Global Address List (GAL)

In a hybrid environment, both on-premises and cloud users should be able to access the same Global Address List (GAL). Verify that the GAL is unified and that users can find and communicate with each other, regardless of their mailbox location.

• Test GAL functionality to ensure that users can locate others in the directory.

• Verify that users can send emails to any other user, regardless of whether their mailbox is on-premises or in the cloud.

After completing the mailbox migration and ensuring that all mailboxes are properly synced and functioning in Microsoft 365, the final step is to complete the transition. In Part 4 of this guide, we will walk you through the last phase of the migration process, which includes updating DNS records, removing hybrid configurations, decommissioning the on-premises Exchange Server, and ensuring that your organization operates fully in Microsoft 365. This part will also cover the steps for providing ongoing support, security, and training for users after the migration.

Updating DNS Records to Redirect Mail Flow

As you transition from Exchange Server 2025 to Microsoft 365, one of the final technical steps is ensuring that all mail flow is directed to Microsoft 365 instead of your on-premises server. This is done by updating your DNS records, which will redirect mail traffic to your cloud-based solution.

Changing the MX Record

The MX (Mail Exchange) record is used to specify the mail servers responsible for receiving email for your domain. To ensure that all incoming email is delivered to Microsoft 365, you need to update the MX record to point to Microsoft’s mail servers.

Log in to your domain registrar or DNS hosting provider’s portal.

Navigate to the DNS management section and find the existing MX record.

Update the MX record to the value provided by Microsoft 365. This typically follows the format: <yourdomain>.mail.protection.outlook.com.

Save the changes and allow time for DNS propagation. DNS changes can take up to 72 hours to fully propagate, so it’s important to plan accordingly.

Once DNS propagation is complete, all email traffic will be directed to Microsoft 365, bypassing the on-premises Exchange Server entirely.

Updating Autodiscover and SPF Records

In addition to the MX record, you must also update the Autodiscover and SPF records to ensure proper email delivery and client configuration.

• Autodiscover Record (CNAME): Autodiscover helps email clients (such as Outlook) automatically configure account settings. Update the Autodiscover CNAME record to point to the correct location in Microsoft 365.

• SPF Record (TXT): The Sender Policy Framework (SPF) record helps prevent your email from being marked as spam. It specifies which mail servers are allowed to send mail on behalf of your domain. Update the SPF record to include Microsoft 365’s mail servers.

Verifying DNS Changes

After updating the DNS records, verify that everything is configured correctly. You can use Microsoft 365’s Domain Verification Tool in the Admin Center or third-party tools like MXToolbox to check that the new MX and Autodiscover records are live and properly configured.

• Send test emails from external accounts to verify that mail is routed to Microsoft 365.

• Use third-party DNS verification tools to confirm that your records are properly updated.

Removing Hybrid Configuration

After you have confirmed that mail flow is directed to Microsoft 365 and DNS records are fully updated, the next step is to remove the hybrid configuration. This step completes the transition to the cloud and ensures that all legacy configurations are removed.

Removing Migration Endpoints and Connectors

Hybrid connectors and migration endpoints are used to facilitate the migration process, but they are no longer needed once all mailboxes have been migrated to Microsoft 365. To remove these:

Log in to the Exchange Admin Center (EAC) in Microsoft 365.

Go to Mail Flow > Connectors and remove any hybrid connectors that were used during the migration process.

Delete migration endpoints to ensure that no legacy connections remain.

Disabling Directory Synchronization (Optional)

If your organization has decided to manage all users directly in Microsoft 365 rather than synchronizing with on-premises Active Directory, you can disable directory synchronization. This is an optional step, but it is recommended if you plan to fully transition to a cloud-only environment.

To disable directory synchronization:

Verify that all user accounts are cloud-only and no longer rely on the on-premises Active Directory.

Use PowerShell or the Microsoft 365 Admin Center to disable directory synchronization.

Uninstall Azure AD Connect from your on-premises server once directory synchronization is stopped.

Disabling directory synchronization means that Microsoft 365 will handle all user management tasks, including authentication and provisioning.

Decommissioning Exchange Server 2025

Once you’ve fully transitioned to Microsoft 365 and removed the hybrid configuration, you can begin the process of decommissioning your on-premises Exchange Server 2025. Decommissioning involves uninstalling Exchange Server and ensuring that no mailboxes, public folders, or mail flow connectors are left behind.

Pre-Decommission Checks

Before decommissioning your Exchange Server, make sure that the following tasks are completed:

Verify that no mailboxes are still on-premises – All user mailboxes should have been successfully migrated to Microsoft 365.

Confirm that no mail flow connectors are still in use – Ensure that email traffic is now directed to Microsoft 365.

Backup or archive any remaining data – If there are any mailboxes, public folders, or other data still on the on-premises server that needs to be archived, make sure it is backed up properly.

Uninstalling Exchange Server

To uninstall Exchange Server 2025:

Go to Control Panel > Programs and Features on the Exchange server.

Select Microsoft Exchange Server 2025 and choose Uninstall.

Follow the uninstall steps provided by the Exchange setup wizard.

After uninstalling Exchange, use tools like ADSI Edit to remove any leftover configuration objects in Active Directory.

Once the server is uninstalled, it’s important to verify that all Exchange-related objects have been removed from Active Directory and that no residual dependencies remain.

Post-Migration Support and User Training

After completing the migration and decommissioning the on-premises Exchange Server, it’s essential to provide ongoing support for users and ensure that they can efficiently use the new Microsoft 365 environment.

User Access Verification

Verify that all users can access their email, calendar, contacts, and tasks in Microsoft 365. Test the following:

• Ensure users can send and receive email without any issues.

• Check that calendar events, contacts, and tasks have been migrated correctly.

• Verify mobile device synchronization, ensuring that users’ smartphones and tablets are properly set up with their new Microsoft 365 accounts.

Permissions and Shared Resources

If users had access to shared mailboxes, calendars, or delegate permissions, ensure that these permissions are restored in Microsoft 365.

• Manually configure any permissions that were not automatically transferred during the migration process.

• Test shared mailbox access and ensure that users can manage shared calendars and email as before.

Providing Microsoft 365 Training

To help your users make the most of their new environment, provide training on how to use Microsoft 365’s features. This includes:

• Outlook for email and calendar management.

• Using OneDrive and SharePoint for file storage and collaboration.

• Collaboration tools such as Teams for messaging and meetings.

Training should be tailored to different user roles, ensuring that each group understands how to maximize productivity with Microsoft 365.

Securing the New Environment

Now that your organization is fully operating in Microsoft 365, it’s critical to ensure that your environment is secure and resilient to potential threats.

Enabling Multi-Factor Authentication (MFA)

One of the best ways to protect your organization’s data is by enabling Multi-Factor Authentication (MFA). MFA adds an extra layer of security to user logins, requiring users to provide a second form of authentication in addition to their passwords.

• Enable MFA for all users in the Microsoft 365 Admin Center.

• Encourage users to set up MFA using their mobile phones, security keys, or authentication apps.

Configuring Conditional Access Policies

Conditional Access policies help manage how users access Microsoft 365 services based on conditions such as location or device compliance. You can use Azure AD Conditional Access to enforce security policies such as:

• Requiring MFA when accessing Microsoft 365 from an untrusted network.

• Restricting access based on device compliance or location.

Data Protection and Compliance

Ensure that your organization meets regulatory compliance and data protection requirements by enabling the following features in Microsoft 365:

• Data Loss Prevention (DLP) policies are used to prevent sensitive information from being shared inappropriately.

• Retention policies to ensure that data is retained for the required period and deleted after it is no longer needed.

• Email encryption protects sensitive communications.

Backup and Disaster Recovery

While Microsoft 365 ensures high data availability, it’s still important to consider third-party backup solutions for additional data protection. These tools can back up emails, files, SharePoint documents, and Teams data, providing an extra layer of security.

Final Documentation and Review

It’s essential to document the entire migration process for future reference, audits, and troubleshooting. This documentation should include:

• The migration timeline and activities.

• Changes made to DNS settings and their verification.

• Errors encountered during migration and their resolutions.

• The final configuration settings in Microsoft 365.

Store this documentation in a secure, cloud-based location such as SharePoint for easy access and sharing with your IT team.

Conclusion

Congratulations! You have successfully migrated from Exchange Server 2025 to Microsoft 365. By following the steps outlined in this guide, you have ensured a smooth transition, including updating DNS records, removing hybrid configurations, decommissioning the on-premises Exchange server, and securing the new environment.

With Microsoft 365 fully operational, your organization is now positioned to take advantage of the robust features and enhanced collaboration capabilities that the cloud has to offer. Going forward, focus on optimizing the environment, ensuring ongoing security, and supporting users in their new cloud-based workflows. Your organization is now better equipped for the future, with increased flexibility, scalability, and efficiency.