Plain-English Explainer
How to Summarize an Email Thread with Copilot in Outlook
Forty unread replies and a meeting coming up is exactly what this button is for.
You open a thread that has been growing in your inbox since yesterday and a meeting about it starts soon. Microsoft 365 Copilot in Outlook can read that whole thread and hand you a summary at the top of the email, with citations that link back to the messages it drew from. This page walks through exactly how to trigger that summary on desktop, web, Mac, and mobile, how to summarize attachments separately, and what to check before you forward the result to anyone.
Microsoft 365 Copilot can summarize a long email thread in Outlook: open the thread and select Summary by Copilot (it may say Summarize instead) at the top of the thread. Copilot reads every message and produces a summary that appears at the top of the email, often with numbered citations you can click to jump to the specific message that supports each point. On Outlook for iOS and Android, select the Copilot icon from the toolbar instead. If the thread has PDF, PowerPoint, or Word attachments, summarize those separately with Summarize a file in new Outlook.
Published
Key Takeaways
- Select Summary by Copilot (sometimes labeled Summarize) at the top of a thread to get an instant summary without opening every message.
- Numbered citations in the summary link directly to the email that supports each point, so you can verify a claim in seconds instead of scrolling.
- Attachments are summarized separately: in new Outlook, use Summarize a file for PDF, PowerPoint, or Word documents attached to the thread.
- On Outlook for iOS and Android, the thread summary lives behind the Copilot icon in the toolbar, not a text button.
- A summary is a starting point, not a substitute for reading the thread yourself before you commit to a decision or forward it to someone else.
- Treat the summary the way you'd treat a colleague's recap: useful for triage, worth checking against the source before it drives a decision.
Where to find the thread summary by platform
| Surface | How you start the summary | Attachment summaries |
|---|---|---|
| Outlook on Windows (new Outlook) | Select Summary by Copilot (or Summarize) at the top of the email thread | Select Summarize a file for PDF, PowerPoint, or Word attachments on the thread |
| Outlook on the web | Select Summary by Copilot (or Summarize) at the top of the email thread | Same entry point as new Outlook when the attachment type is supported |
| Outlook for Mac | Select Summary by Copilot (or Summarize) at the top of the email thread | Follows the same new Outlook experience where available |
| Outlook for iOS and Android | Select the Copilot icon from the toolbar | Open the attachment directly for a file-level summary where supported |
Copy and Paste
Prompts You Can Use Today
Pull out action items before a meeting
Looking at this email thread, list every action item that was assigned to me specifically, who assigned it, and any deadline or date that was mentioned, so I can walk into my next meeting already knowing what I owe people.
Ask this in the Copilot chat pane in Outlook after the thread summary loads, swapping 'me' for the name on the project if you're checking commitments for someone else.
Catch up on a thread you were added to late
I was just added to this email thread partway through. Summarize what has been decided so far, what is still unresolved, and which person seems to be waiting on an answer from me before they can move forward.
Useful when you've been CC'd or looped in after a thread already has dozens of replies.
Check for disagreement buried in a long thread
Did anyone in this email thread disagree with the proposed approach or raise a concern that wasn't fully addressed in a later reply? Quote the relevant line and tell me which person raised it.
Good for threads where a decision was made quickly and you want to make sure a dissenting voice wasn't dropped.
Draft a status update from the thread
Based on this email thread, draft a short status update I can send to my manager that covers where things stand, what the main open question is, and what I need from them, written in a plain and direct tone.
Review and edit before sending; this is a draft, not a finished message.
Reconcile a summary with an attached document
Compare the email thread summary with the attached document and tell me if there's anything the document says that the email conversation doesn't mention, especially around dates, dollar amounts, or responsibilities.
Run this after you've summarized both the thread and the file attachment, so you have both summaries in front of you.
The Basics
What Summary by Copilot actually does to a thread
Before you trust a summary, it helps to know what's happening under the hood.
It reads the whole thread, not just the latest reply
Microsoft 365 Copilot processes every message in the thread you have open, not only the most recent one, so a summary triggered on message forty still accounts for message one.
The summary sits at the top of the email
Instead of opening in a side panel, the summary appears directly at the top of the thread you're already reading, so it stays in view as you scroll back through the original messages.
Numbered citations point back to the source
Where the summary includes numbered citations, selecting one takes you straight to the specific email in the thread that the summary drew that point from.
Attachments get their own summary
A thread summary covers the email text. Files attached to the thread, like a PDF or a Word document, are summarized separately with Summarize a file in new Outlook.
Step By Step
Summarizing a thread on Outlook desktop and web
Open the email thread you want summarized, the same way you normally would. Near the top of the thread, look for Summary by Copilot. Microsoft's own documentation notes the label can also appear as Summarize, so if you don't see the exact first phrase, look for the second one in the same spot before assuming the feature isn't available in your view.
Select it. Copilot works through the thread and produces a summary that appears at the top of the email. For a long thread, give it a moment to finish before you act on what you see, the same way you'd wait for a page to finish loading before reading it.
If the summary includes numbered citations, those numbers are clickable. Selecting one takes you to the corresponding email in the thread, so you can confirm the context around that specific point rather than taking the summary's phrasing at face value. This is the single most useful habit to build when you're using the feature under time pressure: click the citation on the point that matters most to your meeting, read that one message in full, and move on.
The behavior is consistent between Outlook on the web and the new Outlook app on Windows. If your organization has rolled out new Outlook, the entry point and the summary itself work the same way whether you're in a browser tab or the desktop client.
On A Mac
Summarizing a thread in Outlook for Mac
Outlook for Mac follows the same pattern as Windows and the web. Open the thread, find Summary by Copilot (or Summarize) at the top of the email, and select it. There's no separate menu to dig through and no different gesture to learn; Microsoft built this as one consistent entry point across the Outlook clients that support it.
If your Mac is running new Outlook, you'll also have access to Summarize a file for supported attachment types on the thread, the same as on Windows and the web. If the summary button isn't showing up on a particular thread, it's worth checking whether that thread or folder is one your organization has excluded from Copilot processing, which is a common reason the option disappears even though it works everywhere else in your inbox.
On The Go
Summarizing a thread on Outlook for iOS and Android
Mobile works a little differently, which matters if you're trying to catch up on a thread between meetings from your phone. Instead of a text label at the top of the email, open the thread and select the Copilot icon from the toolbar. That's the entry point on both Outlook for iOS and Outlook for Android.
Once you select it, the behavior is the same as on desktop: Copilot reads the thread and gives you a summary, which may include the same kind of citations linking back to individual messages. On a small screen, those citations are especially useful, since they let you jump straight to one message instead of scrolling through dozens of replies with your thumb.
Attachments
What to do when the thread has files attached
A thread summary and a file summary are two different things, and conflating them is the easiest way to miss something important.
Summarize a file is a separate command
In new Outlook, when the email has a PDF, PowerPoint, or Word attachment, you'll see Summarize a file as its own option. It reads the document itself, not the email text around it.
Run both when the file matters to the decision
If a decision in the thread hinges on a number or clause inside an attached document, summarize the thread and the file separately, then compare the two before you act on either one alone.
Watch for attachments the thread summary skipped
A thread summary focuses on the conversation. If someone attached a document three messages back and never explicitly discussed it, the thread summary may not surface it at all, so you still need to check the attachment list yourself.
Before You Forward It
What to verify before you send a summary to someone else
A summary you read yourself and a summary you forward to your manager, a client, or a teammate carry different risk. Reading it yourself, a misread point costs you a few minutes. Forwarding it, a misread point becomes someone else's wrong assumption, and they usually won't go back and check the original thread themselves.
Before you send a Copilot-generated summary onward, click through at least the citations tied to any decision, dollar figure, date, or commitment. These are the details most likely to cause a real problem if the summary paraphrased them loosely or combined two separate statements from different people into one line.
Check who said what. A summary written in neutral, third-person language can flatten the difference between 'I approved this' and 'someone suggested this and nobody objected.' If the thread involves more than two or three people, that distinction often matters more than the content itself.
Check for anything the summary left out entirely, not just anything it got wrong. Summaries are built to compress, and compression means something gets cut. If you know the thread contained a disagreement, a deadline change, or a budget concern, confirm it's reflected somewhere in the summary before you treat its absence as resolution.
If you're forwarding the summary as the record of what was decided, say so explicitly, and say that it was Copilot-generated. That one sentence gives the recipient the context to double-check it themselves if the stakes are high enough to warrant it, instead of assuming it was written and verified by you line by line.
Where This Fits In Your Day
When a thread summary is the right tool, and when it isn't
A thread summary earns its place when you need to triage: deciding whether a forty-message thread needs your attention right now, or whether it can wait until after your next meeting. It's also useful when you're joining a conversation midway and need the shape of the discussion before you can contribute anything useful.
It's a weaker fit when the thread is the actual negotiation, not just a record of one. If people are actively working out terms, assignments, or numbers in real time across the thread, a summary written at one point in time will go stale the moment the next reply lands. In those cases, read the live thread rather than relying on a snapshot summary.
It's also not the right tool for anything where the exact wording matters, like a contractual commitment someone made in an email. Use the citation to go read that exact sentence yourself rather than trusting a paraphrase of it, even a good one.
Reading about Copilot and actually using it fluently at work are two different things
Summarizing one thread is a single skill. The harder part is knowing, day to day, when to trust Copilot's output and when to slow down and check it yourself. The Copilot first week at work course from Nightschool AI walks through real inbox, document, and meeting scenarios in Outlook, Word, Excel, Teams, and OneNote so you build that judgment early, not after a mistake teaches it to you.
Frequently Asked Questions
Why don't I see Summary by Copilot on some email threads?
The label can appear as either Summary by Copilot or Summarize depending on your version of Outlook, so check for both before assuming it's missing. If neither appears, it may be that your organization has restricted Copilot on certain folders or mailboxes, or that the specific thread type isn't supported for summarization yet.
Does the Copilot summary work the same way on Outlook for Mac as on Windows?
Yes. Outlook for Mac uses the same entry point: select Summary by Copilot (or Summarize) at the top of the thread. The summary behaves the same way, appearing at the top of the email with the same citation behavior when citations are included.
How do I summarize an email thread on my phone?
On Outlook for iOS and Android, open the thread and select the Copilot icon from the toolbar rather than looking for a text button. The summary that results works the same as on desktop, including any citations linking back to specific messages.
Can Copilot summarize an attachment, not just the email text?
Yes, but it's a separate action. In new Outlook, if the email has a PDF, PowerPoint, or Word attachment, select Summarize a file to get a summary of the document itself, independent of the thread summary.
What are the numbered citations in a Copilot email summary for?
When a summary includes numbered citations, selecting one takes you directly to the corresponding email in the thread that supports that point. They're the fastest way to verify a specific claim in the summary without scrolling through the whole thread manually.
Is it safe to forward a Copilot-generated summary without reading the whole thread myself?
It depends on the stakes. For low-stakes triage, forwarding a summary as-is is usually fine. For anything involving a decision, a dollar figure, a date, or a commitment someone made, click through the relevant citations first and consider telling the recipient the summary was Copilot-generated.
Why does the thread summary seem to miss something that was discussed in an attachment?
The thread summary is built from the email conversation itself, not necessarily the content of attached files. If a document was attached but not explicitly discussed in the replies, run Summarize a file separately to capture what's inside it.
Does summarizing a thread change or delete any of the original messages?
No. The summary appears at the top of the email as an added view of the thread; it doesn't alter, delete, or replace the original messages, which remain exactly as they were sent.
What should I check before using a Copilot summary to brief someone before a meeting?
Confirm who said what, not just what was said, since a summary can flatten different people's statements into one neutral line. Also check whether anything you know was discussed, like a disagreement or a deadline change, is actually reflected in the summary rather than dropped during compression.
A meeting coming up is exactly when this skill pays off
Knowing the button is one thing. Knowing when to trust it is another.