Grouping in New outlook

New Outlook can group messages by conversation, but Microsoft’s documented custom field-based Group By procedure—such as grouping by sender, category, size, or importance—is for classic Outlook.

Group messages by conversation in new Outlook

1. Select Settings (gear icon).
2. Go to Mail → Reading Pane.
3. Under Message organization, choose:
   • Group messages by conversation
   • Group messages by branches within conversations
   • Do not group messages

A conversation collects messages from the same email thread and subject line. ^1^

If you mean classic custom grouping

Classic Outlook provides View → View Settings → Group By, where you can group by fields such as Categories, Subject, Size, or Importance, and optionally add subgroup levels. ^2^

The available Microsoft documentation does not confirm an equivalent custom Group By view in new Outlook. If that exact view is essential, use classic Outlook for it. You can also request the capability in new Outlook through Help → Give Feedback; Microsoft directs new-Outlook feature requests there. ^3^

References

1. View email messages by conversation in Outlook
2. Group or ungroup messages in your Inbox | Microsoft Support
3. Feature comparison between new Outlook and classic Outlook