Instead of hunting down all messages belonging to a conversation in multiple folders (Inbox, archive, Sent Mail and what not) have Outlook do the organizing and read together messages that belong together.
View Mail Grouped by Conversation Thread in Outlook
To see mail arranged by conversation in Outlook:
- Go to the View tab in the main Outlook window's ribbon.
- Make sure Show as Conversations is checked in the Conversations area.
- Click All folders to group all messages — including those from your "Sent Items" folder, for example; click This folder to see only messages from the current folder threaded.
Now, conversations will show an arrow next to their left in the message list. Click the arrow to expand and collapse threads.
Unread messages appear bold; any collapsed conversation with at least one unread message will appear bolded, too.