How to Use Microsoft 365 Copilot to Summarize a Long Thread in New Outlook

Reading a long email thread with dozens of replies can take too much time. Microsoft 365 Copilot in the new Outlook for Windows can read the entire conversation and produce a short summary of the key points. This article explains how to use the Copilot Summarize button, what prerequisites you need, and how the summary handles nested replies and attachments. You will learn the exact steps to generate a thread summary in seconds.

What the Copilot Summarize Feature Does and What You Need

The Copilot Summarize feature in the new Outlook reads every message in the currently selected email thread. It then produces a short paragraph or a set of bullet points that capture the main decisions, questions, and action items. The summary does not include every reply verbatim. Instead, Copilot extracts the most relevant information and presents it in plain English.

The feature works on any folder that contains conversations: Inbox, Sent Items, Archive, or a custom folder. It also works on threads that include forwarded messages and replies with inline comments. Copilot does not read attachments such as PDFs or Word documents. It only processes the text inside the email bodies.

Prerequisites

To use the Summarize button you need three things. First, you must have a Microsoft 365 Copilot license assigned to your account. This license is separate from a standard Microsoft 365 E3 or E5 subscription. Second, you must be using the new Outlook for Windows. The classic Outlook does not have the Copilot Summarize button. Third, the thread must contain at least 10 messages. Threads with fewer messages show the Summarize button as grayed out or do not display it at all.

Steps to Summarize a Long Thread in New Outlook

1. Open the new Outlook for Windows
   Launch the app from the Start menu or the taskbar. Make sure you are signed in with your work or school account that has a Copilot license.
2. Navigate to the folder that contains the long thread
   Click the Inbox folder or any other folder where the conversation is stored. You can also use the search bar at the top to find the thread by subject or sender.
3. Select the thread in the message list
   Click the thread once. Do not double-click to open it in a separate window. The thread must be highlighted in the reading pane or the message list.
4. Locate the Copilot button in the ribbon
   Look at the top of the Outlook window. The ribbon contains a section labeled Copilot. The button shows a sparkle icon with the text Summarize. If the ribbon is collapsed, click the ellipsis (three dots) to expand it.
5. Click the Summarize button
   Click the Summarize button once. Copilot begins processing the thread. A loading indicator appears next to the button. Processing time depends on the number of messages. A thread with 50 messages typically takes 5 to 10 seconds.
6. Review the summary that appears above the message list
   After processing, a box appears at the top of the reading pane. The box contains the summary text. It includes a header that says Summarized by Copilot. The summary is read-only and cannot be edited directly.
7. Copy or dismiss the summary
   Click the Copy button inside the summary box to copy the text to your clipboard. You can paste it into a reply, a meeting invite, or a document. Click the X button to dismiss the summary. The summary does not persist after you dismiss it. To see the summary again, click the Summarize button again.

The summary box also contains a thumbs-up and thumbs-down icon. Microsoft uses this feedback to improve the Copilot model. You can submit feedback without leaving the Outlook window.

Common Limitations and Things to Avoid

The Summarize button is grayed out or missing

If the button appears but is not clickable, the thread has fewer than 10 messages. Select a longer thread to activate the button. If the button is completely missing, your account does not have a Copilot license. Contact your IT administrator to verify the license assignment. The button also does not appear in the classic Outlook. Switch to the new Outlook for Windows by toggling the Try the new Outlook switch in the top-right corner.

The summary does not include attachment content

Copilot only reads text inside email bodies. Attachments such as PDFs, Word documents, Excel files, or images are not scanned. If a critical decision is documented inside an attached file, the summary will not mention it. Open the attachment manually to verify important details.

The summary misses context from deeply nested replies

Copilot summarizes the entire thread but may omit minor replies that do not contain new information. If you need to see every individual reply, scroll through the thread manually. The summary is a compressed version, not a full transcript.

The summary appears in a different language

Copilot generates the summary in the same language as the majority of the thread. If the thread contains mixed languages, the summary may switch between languages or default to your Outlook display language. To change the display language, go to File > Options > Language.

Item	New Outlook with Copilot	New Outlook without Copilot
Summarize button visible	Yes, on threads with 10+ messages	No
Thread length requirement	10 messages minimum	Not applicable
Attachment content included	No	Not applicable
Summary language	Matches majority thread language	Not applicable
Copy summary to clipboard	Yes, via Copy button	Not applicable

The Summarize button is a time-saving tool for long email threads. You can generate a summary on any thread that meets the 10-message threshold. After reading the summary, you can reply to the thread with a quick acknowledgment without reading every previous message. For deeper analysis, consider using Copilot Chat and asking specific questions about the thread content.