Well, it’s not perfect, but it may be useful to some.
- Create a new search folder and name it All Mail. Don’t specify any criteria, and select the folders you want (should be only inbox and sent items folders. You may also want to uncheck the “include subfolders“ option.)
- Go to All Mail and wait for it to finish searching (it may take a while, but it’s a one-time thing.)
- Click the Arrange By column and select Conversation.
Gmail does this better, as conversations are grouped even when the subject changes, and it only groups a conversation from one person, whereas Outlook may include a mixture in a single group.