Hi,
Please confirm that if configuration mentioned in "Create an Outlook.com group and add members" are helpful:
https://support.microsoft.com/en-us/office/create-an-outlook-com-group-and-add-members-cb051614-a202-40b2-bd30-04eca896aeec
Above is my personal suggestion and just for your reference. Based on your description, your question is mainly about outlook account and group mails, it is beyond the support scope of windows-group-policy, I would recommend you to post on Microsoft Community Forum - Outlook.com, and relate product supporter may provide you more suggestion:
https://answers.microsoft.com/en-us/outlook_com/forum?sort=LastReplyDate&dir=Desc&tab=All&status=all&mod=&modAge=&advFil=&postedAfter=&postedBefore=&threadType=All&isFilterExpanded=false&page=1
Thank you for your understanding.
Best Regards,
Eve Wang