View email messages by conversation in Outlook - Microsoft Support
View messages grouped by conversation in new Outlook. At the top of the page, select Settings > Mail > Layout. Under Message organization, choose either of the options depending on whether you want to show email grouped by conversation or as individual message.
Applies To:
Outlook for Microsoft 365, Outlook 2021, Outlook 2019, Outlook 2016, Outlook Web App for Office 365 Small Business, Outlook.com, New Outlook for Windows