Using New Outlook on Windows for personal email, calendar, and contact management
I understand that New Outlook does not recognize your email group, so you cannot send messages to it.
Please open New Outlook > People > All contact lists and check whether the group appears there. If it is missing, try creating a new contact list directly in New Outlook and add the members again. Then, compose a new email and select the list from the suggestions in the To field. Problems with address lists and groups recommends this test to check whether the contact list is syncing correctly.
If the group name appears in red, try expanding the group using the + icon before sending. As a temporary workaround, you may also send the message from Classic Outlook.
Could you also confirm whether you are using a personal Outlook.com account or a work/business Microsoft 365 account?
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