What Is an Email Thread? How Gmail and Outlook Group Emails

An email thread is an email and every reply to it, shown together as one conversation. Ask a caterer for a quote, get a price and confirm the numbers: your inbox shows one entry, not three separate emails.

Gmail and Outlook decide what belongs together from the subject line and from reference headers that each reply carries. Change the subject, or write a fresh email instead of replying, and the next message begins a separate thread.

Email thread meaning: thread, chain or conversation

The three words name the same thing: an opening email and the replies that followed it. Microsoft's help for Outlook puts it this way: "You might know a Conversation by the term email thread" (Microsoft Support).

"Conversation" is the word on the settings screens of both apps. "Email chain" is the casual word, and people often mean the quoted history at the bottom of a long reply.

In British offices you will also hear "email trail", usually when the messages serve as a record of what was agreed.

How email threading works: the headers behind every reply

Nearly every email carries a hidden ID, and each reply copies it into two more header lines. The rules come from an RFC, short for Request for Comments: the numbered technical documents that define email. RFC 5322, section 3.6.4 covers all three lines:

• Message-ID: a unique ID for one message, created by the sender's mail system. It looks like an address: letters and numbers, an @ and a domain.
• In-Reply-To: the Message-ID of the email being answered, which the standard calls the parent.
• References: everything in the parent's References line, plus its Message-ID at the end. Each reply adds one ID, so the list traces the conversation back to where it began.

The reply also puts "Re:" in front of the subject. The standard explains it as an abbreviation of the Latin "in re", meaning "in the matter of" (section 3.6.5).

A three-email thread, header by header

The thread below is our own made-up example: a small design studio orders lunch from a caterer. Its addresses end in .example, a reserved name that no real business can own (RFC 2606). Only the lines that link the messages are shown; a real header holds many more.

Message 1: the studio asks for a quote

From: [email protected]
To: [email protected]
Subject: Lunch for 30 on 12 November
Date: Tue, 3 Nov 2026 09:15:12 +0000
Message-ID: <[email protected]>

Message 2: the caterer replies with a price

From: [email protected]
To: [email protected]
Subject: Re: Lunch for 30 on 12 November
Date: Tue, 3 Nov 2026 11:02:40 +0000
Message-ID: <[email protected]>
In-Reply-To: <[email protected]>
References: <[email protected]>

Message 3: the studio confirms and copies in its accounts team

From: [email protected]
To: [email protected]
Cc: [email protected]
Subject: Re: Lunch for 30 on 12 November
Date: Tue, 3 Nov 2026 14:30:05 +0000
Message-ID: <[email protected]>
In-Reply-To: <[email protected]>
References: <[email protected]>
 <[email protected]>

This is how the studio's mail app joins the three:

1. Message 2 arrives. Its In-Reply-To holds the ID of message 1, so the app files it under message 1.
2. Message 3 goes out. Its References lists both earlier IDs, oldest first, which is enough to rebuild the whole chain.
3. The app compares subjects with the "Re:" set aside. All three read "Lunch for 30 on 12 November", so the thread holds.
4. The accounts team sees less. Their mailbox received only message 3, so their copy of the thread starts there. Messages 1 and 2 reach them only if they are quoted below it.

Now suppose the studio had renamed message 3 "Invoice for the 12 November lunch". Gmail would list it as a new conversation, although References still points back (Gmail Help). Microsoft describes an Outlook conversation as messages "in the same thread with the same subject line", so the renamed reply falls outside it too.

To see these lines on a real email, open its full header. Our post on SMTP and the Received lines shows where to find it in Gmail and new Outlook.

How Gmail and Outlook group emails into a thread

Both apps start from the subject, then add one check each: Gmail reads the References header, and Outlook reads an index of its own.

	Gmail	Outlook
What it reads	The subject, and the References header when there is one: it has to point to an earlier message in the thread	The subject without RE: or FW:, carried in a Thread-Topic header, and a conversation index in a Thread-Index header; Outlook adds both to every email it sends
Where the newest email sits	At the bottom of the conversation	At the top in classic Outlook (fixed) and in Outlook for Mac
When a new thread starts	When the subject changes or the thread passes 100 emails	When the subject changes
Extra layout	None	Group messages by branches within conversations, in new Outlook and on the web

Sources: Gmail Help, Google Workspace Updates, 29 March 2019, Microsoft Support, Outlook for Mac, Microsoft Learn, Microsoft Open Specifications on Thread-Topic and Thread-Index.

Gmail's reference rule dates from March 2019. Before it, two emails from one sender with a matching subject, sent within a week, joined one conversation.

Now they stay apart unless one references the other. Two order confirmations from a shop, sent days apart under one subject, remain two entries.

A branch appears when two people answer the same message separately. When the caterer and the accounts address both answer message 3, each reply points to it, but neither points to the other. Outlook's branches layout is named after this case.

How to turn conversation view on or off

Both apps group emails by default, and the switch sits in a different place in each. The steps follow the Google, Apple and Microsoft help pages as of October 2026.

Gmail on a computer

1. Click Settings at the top right, then See all settings.
2. Scroll to the Conversation view section and select Conversation view on or Conversation view off.
3. Click Save changes.

Switching it off also hides Gmail's nudges, the prompts at the top of the inbox to reply or follow up.

Gmail on Android

Tap Menu, then Settings, then your account address. Under General, tick or untick Conversation view. For a non-Gmail account added to the app, the setting is Email conversation view under General settings.

Gmail on iPhone and iPad

Open the app menu and tap Settings. Under Inbox, tap Inbox customisations and use the Conversation view switch.

Mail on iPhone

Go to Settings, then Apps, then Mail, and turn Organize by Thread on or off.

New Outlook for Windows and Outlook on the web

Go to Settings, then Mail, then Reading Pane. Under Message organization, choose Group messages by conversation, Group messages by branches within conversations or Do not group messages.

Classic Outlook on Windows

From any mail folder, select View, then Show as Conversations. Outlook asks whether the change applies to all mailboxes or only this folder.

Outlook for Mac

In the View menu, untick Show as Conversations.

Why part of an email thread is missing

A thread that looks incomplete usually has a missing piece somewhere else: in another folder, another thread or another person's mailbox.

What you see	Likely reason	What to do
Your own replies are missing in classic Outlook	The conversation shows only the current folder	View, Conversation Settings, then Show Messages from Other Folders, which also brings in Sent Items
A message you deleted or flagged as junk has gone from the thread on a Mac	Outlook for Mac leaves Junk E-Mail and Deleted Items out of conversations	Open that folder directly
A reply sits on its own line	The sender changed the subject or wrote a fresh email	Search for a word from the original subject
A colleague's copy starts halfway through	They joined later, as in step 4 of the example	Forward the earlier message they need

Reply or Reply all in a long email thread

Reply answers only the sender. Reply all also includes everyone on the To and Cc lines of the message you are answering (Microsoft Support).

When the sender has set a Reply-To address, your reply goes there rather than to From (RFC 5322, appendix A.2). Bounce reports go to the address recorded in the Return-Path header, so replies and failed-delivery notices can land in different mailboxes.

Reply all takes its list from one message, not from everyone who has ever been on the thread. In the example, [email protected] joined in message 3. If someone at the studio later clicks Reply all on message 2, the caterer's reply, that address drops out.

Hidden copies add a separate trap: a Bcc recipient who clicks Reply all shows everyone else on the email that they had a copy.

Each reply usually quotes the messages before it. Someone you add later, such as an outside supplier, can scroll down and read the whole history, including notes meant only for colleagues. Delete the quoted text you do not want them to see before you send.

Attachments behave differently. Reply and Reply all leave out the files from the earlier message, while Forward keeps them, as Microsoft's Outlook help notes. A colleague added halfway through gets the quoted words but not the caterer's menu PDF, so forward the message that holds the file.

When to start a new email thread

Start a new thread when the topic, the people or the purpose of the conversation changes. Three common moments:

• A new topic: the lunch invoice deserves its own subject, so anyone can find it later by searching.
• New people: someone joining from outside should not inherit weeks of quoted replies.
• A very long thread: Gmail splits a thread once it passes 100 emails anyway, and nobody scrolls that far.

Use Compose in Gmail or New email in Outlook: a renamed reply still carries the old recipients and quoted text. Open the fresh email with one line that links back, such as "Following our lunch order of 3 November".

How to stop getting replies from an email thread

You cannot stop people replying, but you can keep a thread's replies out of your inbox: mute it in Gmail or ignore it in classic Outlook.

Mute a thread in Gmail

Tick the conversation, click More, then Mute. With keyboard shortcuts turned on, you can press M instead.

Later replies skip your inbox and go to the archive with the label Muted (Gmail Help). A reply still reaches your inbox when it:

• is sent only to you
• comes through a Google Group you belong to
• adds you to To or Cc

To find muted threads, search for is:muted.

Ignore a conversation in classic Outlook

Select the conversation, then on the Home tab select Ignore and confirm with Ignore Conversation. Outlook removes the existing messages and sends every future reply straight to Deleted Items (Microsoft Support).

Do email campaigns create a thread?

No: an email platform sends each contact a separate email, so recipients never share a thread. Each MailGraf campaign copy has its own Message-ID, one address in To and no In-Reply-To or References lines (our sending code, October 2026). Each reply comes back on its own, to the campaign's sending address or its Reply-to address. If a customer update has ever turned into a Reply all chain, the pricing page shows the plan for sending campaigns.