Dear LarsE._47,
Good day! Thank you for posting to Microsoft Community. We are happy to assist you.
Based on your description, you are having issue changing the SMTP settings for your IMAP account in New Outlook.
After thorough research, testing on my side, and consulting, it was concluded that it is currently not feasible to set up IMAP in new Outlook. This feature is now in development.

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Reference: New Outlook for Windows (microsoft.com).
We apologize if we are unable to meet your needs; However, thank you for bringing this to our attention, we will certainly raise your concerns to our product team and hope that this can be included in future updates.
In the meantime, would please vote on this information that will help other customers in our community who are trying to set up IMAP account in new perspectives. Thanks in advance!
For other concerns, don't hesitate to add your post on the Microsoft Community Team.
Thank you for choosing Microsoft as your business partner.
If there is any update or I misunderstand you, please feel free to let me know. I will continue to assist you. Your understanding and co-operation are highly appreciated.
Thanks for your precious time. Have a nice day.
Sincerely,
Sherry | Microsoft Community Moderator