Coexistence is when HCL Notes and Microsoft 365 run side by side for a period, for example mail in Microsoft 365 while Notes applications keep running on Domino.
Coexistence means that HCL Notes and another platform, typically Microsoft 365, run side by side for a period. A common setup is mail and calendar in Microsoft 365, while the company's own Notes applications keep running on Domino. Then you do not have to move everything in one big bang.
How does it work?
For coexistence to work, a few things must be in place:
- Mail flow: Mail must reach the right inbox, whether the user is on Notes or Microsoft 365.
- Calendar: Users on both platforms must be able to see free and busy time and book meetings.
- Address book: Everyone must be able to find each other, no matter where they are.
- Access to applications: Users who live in Outlook must still reach the Notes systems they need, for example with the Notes client, HCL Nomad in the browser, or through the Domino REST API.
Coexistence in practice
Many companies use coexistence in one of two ways. Some use it for a few months while they move users in groups. Others use it for years, because their Notes applications work well and there is no reason to rebuild them. Both are fine. The important thing is that it is a choice and not an accident.
Why does it matter?
A Notes migration in one go is risky. Coexistence lets you move step by step, test along the way and keep the business running. You can move or close the applications one at a time, when it suits you. Read more about Lotus Notes migration to Microsoft 365, and about whether you should stay or move in our article on Lotus Notes in 2026.