Many users are aware of Apple Mail's message threading feature, which highlights related email messages within the Inbox. However, many people don't know how to view both sent and received messages within a thread at once. To do so, first enable Message Threading under Mail's Viewing Preferences. Then, Command-click both the mailbox containing your threaded messages, and your Sent box. Now you can view both sent and received messages within the thread simultaneously.
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From Sharon Zardetto