Think of a thread summary like the notes a colleague hands you on the way into a meeting. They save you the reading, and you would still glance at the original before you repeat anything from them to your manager. Microsoft 365 Copilot in Outlook works the same way.
What you can use depends on your license and your organization. Microsoft notes that if you don't see Copilot in Word, Excel, PowerPoint, or OneNote, it might not be included with your subscription or available based on your organization's settings. The apps show in-product labels to help you identify your Copilot experience, and your IT admin can tell you which license you have.
Open the conversation and select Summary by Copilot at the top of the email thread (or it might say Summarize). Copilot scans the thread for key points and writes a short summary above the emails. That summary may include numbered citations, and selecting one takes you straight to the email it came from. Those citations are your checking tool, so get in the habit of using them.
Copilot can also help you write the reply. The email you draft can be a new conversation, a reply, or a forward. Once Copilot drafts it, you can change the length or tone, or run the same prompt again, and you edit the draft as needed before you select Send.
A drafting request works best when it names three things: who the email is for, what you are committing to, and by when. "Reply to Dana" leaves Copilot guessing. "Reply to Dana, confirm I will review the revised terms by end of day, two sentences" gives it everything it needs.
Here is the honest limit. A summary picks key points, which means it also leaves things out. A side comment halfway down the thread that moves the deadline might not make the cut, and the summary will read just as confidently without it. You are the one whose name is on the reply, so open the emails the citations point to before you commit to anything.
Summarize the thread, open the cited emails that matter, then draft a reply that says who, what, and when. The source emails are the record and the summary is your shortcut into them.