A Lotus Notes to Office 365 migration moves each user’s Notes mail file, calendar, contacts and to-dos, plus shared mail-in databases, from HCL Domino into Exchange Online mailboxes in Microsoft 365. Microsoft provides an assessment tool for sizing the move, but not a built-in way to migrate the data itself, so the migration runs through third-party migration tooling, with a period of coexistence so Notes and Outlook users can keep working together. Domino applications are a separate workstream.
This article covers the mail and calendar side of the move: what migrates, how a phased Notes migration to Exchange works, how to size it and the mistakes that cause trouble. It is part of our HCL Domino consulting and support resources. If you are still deciding whether to leave Domino at all, start with the Domino migration playbook for CIOs.
Mail is the easy part. The planning effort goes into the items that do not map one to one:
| Notes / Domino item | Exchange Online equivalent | What to watch |
|---|---|---|
| Mail files (NSF) | User mailboxes | Folder structure, read status and very large mail files |
| Calendar entries and meetings | Mailbox calendars | Recurring meetings and exceptions, and meetings organized by users who migrate in different waves |
| Rooms and resources | Room and equipment mailboxes | Booking rules and delegates must be rebuilt |
| Personal address books | Mailbox contacts | Often stored locally on the user’s workstation |
| Domino Directory groups | Distribution lists or Microsoft 365 groups | Nested groups and groups used for application security |
| Mail-in databases | Shared mailboxes | Ownership and who needs access |
| Local and server archives | Online archive mailboxes or retention policies | Local archives are easy to miss in discovery |
| Notes-encrypted messages | Readable mail only after decryption | Messages encrypted with a user’s Notes ID must be decrypted, or they stay unreadable |
| Doclinks | Links to migrated content or a retained Domino server | Links to applications break if the application is retired |
| Domino applications | Not handled by mail migration tools | Plan them separately |
Most Notes migrations to Exchange now target Exchange Online in Microsoft 365. Some regulated organizations still move to on premises Exchange Server. The migration method is similar; the differences are mainly in infrastructure and licensing.
| Factor | Exchange Online (Microsoft 365) | Exchange Server (on premises) |
|---|---|---|
| Infrastructure | Hosted by Microsoft | Your servers and data center |
| Identity | Microsoft Entra ID, usually synchronized from Active Directory | Active Directory |
| Licensing | Per user subscription | Server and client access licenses |
| Common reason to choose | Cloud first mandate, Teams and Microsoft 365 adoption | Data residency or isolated network requirements |
Unless every mailbox moves in one weekend, part of the organization is on Notes while the rest is on Outlook. Coexistence covers three things:
Coexistence is also what lets Domino applications keep running after mail has moved. Applications that send notifications or route approvals by mail need to keep reaching migrated users.
Migration time is driven by data volume and the throughput your tooling achieves in your environment. The figures below are illustrative; use the throughput measured in your own pilot.
| Input | Value |
|---|---|
| Users | 2,000 |
| Average mail file size | 3 GB |
| Total mail data | 2,000 × 3 GB = 6,000 GB |
| Throughput measured in pilot (example) | 50 GB per hour |
| Bulk migration time | 6,000 GB ÷ 50 GB/h = 120 hours |
| Recent mail moved at cutover (last 30 days, about 5%) | 300 GB ÷ 50 GB/h = 6 hours |
That is why most projects pre-stage older mail in the weeks before each wave and move only the recent delta at cutover. Users switch to Outlook with their full history while the cutover window stays short.
Moving Lotus Domino to Office 365 for mail does not move your Notes applications. They can stay on a smaller, modernized Domino environment, be rebuilt on other platforms, or be retired. Our article on migrating complex Notes applications explains how to classify and plan them, and the enterprise Microsoft 365 migration checklist covers the wider Microsoft 365 rollout.
Planning a Lotus Notes to Microsoft 365 migration?
RockTeam has delivered Domino migration projects on and off the platform for decades, including coexistence and application strategy. RockTeam is an official HCL Business Partner. Start with a no cost assessment.
See Email and Data Migration ServicesMicrosoft provides the Online Notes Inspector (MONTI) to assess and size Notes mail data, but the migration itself is performed with third-party migration tools.
Yes. A coexistence setup routes mail between Domino and Exchange, shares free/busy information and keeps both directories in sync until the last users move.
It depends on data volume, the number of waves and the throughput measured in your pilot. Pre-staging older mail keeps each cutover window short, while the overall project typically runs over several waves.
Messages encrypted with a user’s Notes ID must be decrypted, using that ID, before or during migration. Otherwise they remain unreadable in Exchange.
No. Many organizations move mail to Microsoft 365 first and keep applications on Domino, then rebuild, replace or retire them on their own schedule.
