Enabling Conversations View
By default, when you add most account types to Outlook, your messages are displayed in Conversations. A conversation includes all messages in the same thread with the same subject line.
1. From any mail folder, or the mail folder you want to apply this change to, select the "View" tab on the Ribbon.
2. Check the "Show as Conversations" box.
IMPORTANT NOTE: In some versions of Outlook, you may need to click the "Current View" button under the "View" tab to see this option.
3. Select "All Mailboxes" or "This Folder."
4. Conversations are identified by a triangular expand/collapse icon.
You can click that icon to view the entire Conversation, and again to collapse it.
Conversations are sorted with the newest message on top, and this order cannot be changed. Additionally, the Conversation will move to the top of your Inbox when you receive a new message that is part of the exchange.
Conversations that include unread messages will have a bold subject and the count of unread messages appear next the subject line. Any unread messages will have a bold heading when you expand the Conversation.
Changing Conversation Options
Some users may opt to adjust their options to better organize their messages.
1. Select the "View" tab, and then click "Conversation Settings."
NOTE: Again, depending on your version of Outlook, you may need to click the "Current View" button under the "View" tab to see this option.
2. A variety of options will be shown.
Show Messages from Other Folders:
Use this option to display messages in the conversation that have been moved to other folders as well as messages you've sent that are stored in your Sent Items folder.
Show Senders Above the Subject:
Selecting this option shows the sender names at the top of the conversation rather than the conversation subject.
Always Expand Selected Conversation:
This option applies only to the currently selected conversation. The conversation selected when you check this box will always appear expanded.
Use Classic Indented View:
Classic indented view shows messages in the conversation indented based on their position within the conversation.
Reference:
View email messages by conversation in Outlook - Microsoft Support (https://support.microsoft.com/en-us/office/view-email-messages-by-conversation-in-outlook-0eeec76c-f59b-4834-98e6-05cfdfa9fb07#PickTab=Classic_Outlook)