Hello @Johannes Kingma,
Thank you for reaching out to Microsoft Q&A Forum.
At this time, Mail Merge in Word depends on MAPI integration, which is only available in the classic Outlook for Windows. The New Outlook does not currently support this feature, so when you start a Mail Merge, Word will attempt to open Outlook (classic). This is expected behavior.
We understand this may cause inconvenience, and we truly appreciate your patience as we continue improving the New Outlook experience. If you need to use Mail Merge, please switch back to Outlook (classic): In Outlook, go to Settings and turn off “Try the new Outlook”.
Microsoft is aware of this limitation and may consider adding support in the future, but there is no official timeline yet. Your feedback is valuable as we work to enhance the New Outlook experience.
The best way to ensure this gets prioritized by the product team is to submit feedback through the official Microsoft Feedback Portal. The more votes and comments an item receive, the higher its visibility and priority. Here’s the link: https://feedbackportal.microsoft.com/feedback
If you share the link to your feedback here, others in the community (including myself) can upvote it to help bring more attention to the issue.
I totally understand your intention to keep the thread open and hear from more users who might be facing the same issue. I strongly encourage you to mark an “Accepted Answer” for now. Doing so will pin the thread to the top, making it easier for others to find when searching with related keywords. It also helps highlight the discussion for users who might benefit from it, and of course, the thread remains open for anyone to join and share their thoughts.
Thank you again for your patience and raising this important topic, engaged member of the community.
Warmest Regards.