With email threading all email replies that relate to each other are threaded into a single conversation. This allows teammates to view the context about an email thread, together, in one place, rather than needing to split their attention across multiple conversations. Email threading also prevents email threads going through workflows multiple times or getting assigned to different teammates who may not have the right context.

There are specific pieces of information on an email that we use to determine whether or not an email should be threaded into an existing conversation. All emails contain pieces of metadata called headers, and we use specific headers in emails to determine when and how to thread an email:

Message-ID: Every email sent has a unique identifier and that is put into the Message-ID header. It typically looks somewhat like an email address, e.g. <1234567890@example.com>. The email provider that sends the email generates this.
In-Reply-To: When you send a reply to an email, the email provider typically puts the Message-ID of the email you’re replying to into the In-Reply-To header.
References: Similar to the In-Reply-To header this typically contains the Message-ID of the email that was replied to, but it can also contain some or all of the Message-IDs for emails from earlier in the thread.

For emails you have received in Intercom you can view these headers by hovering over the email icon on the bottom left-hand corner of the message and clicking on ‘View raw email’.

When an email is received into your workspace, we take its References and In-Reply-To headers. We use those to look for a conversation in your workspace containing an email with a Message-ID that’s referenced by those headers. If we find one, we thread the new email into that conversation.

External senders

In some circumstances, an email will be threaded into a conversation from an ‘external sender’. This happens when someone who hasn’t been included in any of the emails so far sends an email that references an email from the conversation in its In-Reply-To or References headers.

This can happen when a participant in the conversation Bcc’s someone or forwards an email from the conversation to someone. If that person replies and one of your workspace addresses or forwarding addresses in the To or Cc fields, we add their reply into the same conversation. This allows you to maintain context about the same email thread all together in the same conversation.

Their reply will contain a warning banner to tell the teammate that the person wasn’t previously seen in the conversation, and they were not CC’d by another participant.

The sender won’t be added as a participant into the conversation. The teammate can choose to add them as a participant in the conversation so that they can be included in future replies.

Why is the email reply not threading into an existing conversation?

Some email clients will strip out some of all of the headers from emails which Intercom uses to thread replies into a conversation. When this happens we don’t have a reliable way of finding the conversation to thread the email into. Email clients and servers have a wide range of features and behaviours that may lead to this happening.

One other circumstance where an email will split into a new conversation applies to conversations that started on the Messenger, and later had one or more participants added by email. The user in the Messenger will be given an alias email address when their replies are sent to the other participants of the conversation. That alias email address looks like this: alice.jones.onchat@examply.intercom-mail.com. If one of the conversation participants removes this email address from the To or Cc fields when replying to the conversation, their reply will split out into a new conversation.

Why did an email thread into a conversation when I didn’t expect it to?

Users may forward emails sent on a conversation to other people, or Bcc others into their replies. If those people reply and include one of your workspace or forwarding email addresses as a recipient, it’s likely that the relevant References or In-Reply-To headers will be present, and we’ll thread their reply into the conversation. You can read more about this behaviour in Manage group conversations via the Help Desk.

Some email clients split emails into new threads when the subject line changes in a meaningful way. This doesn’t happen in Intercom.

Why are emails not threading in my email client?

Email clients and servers have a wide range of features and behaviours that may lead to this happening. Some have explicit settings for enabling and disabling threading, so it is worth checking those settings if they exist in your client. Email clients also have varying behaviour in terms of which email headers need to be present in order for emails to thread together, or they may split conversations based on subject line or recipient changes.

Old behavior (conversations older than 12th July 2023)

Previously, in email conversations we set the ReplyTo address for each email to a unique address for each conversation that looks like this: n+u_xxxx@examply.intercom-mail.com

You can now send and receive emails from the actual email addresses of conversation participants, and reply directly to each participant in the conversation. This allows you to clearly see who you're talking to and add control over who will receive your message by allowing you to remove people you don’t want to send to, or add more people in CC.

	Old behavior	New behavior
When recipients change	When a user replies to an email conversation and removes one or more of the other users who received the original email, their reply will split out into a new conversation. E.g. Alice emails support@example.com (Intercom) and CCs Bob and Carol. Bob replies to Alice and support@example.com, but removes Carol from CC. His email creates a second conversation in Intercom.	Email replies to a conversation that have been sent to only a subset of participants will be added to that conversation instead of starting a new one. E.g. Alice emails support@example.com (Intercom) and CCs Bob and Carol. Bob replies to Alice and support@example.com, but removes Carol from CC. Bob’s email is threaded into Alice’s conversation in Intercom. Though Carol is a participant of the conversation she doesn’t receive Bob’s email.
Email headers and replying	Emails sent on a group conversation have an ‘n+u’ email address in the ReplyTo header. When the user replies, they reply to that address and their reply is added to the conversation via the Help Desk for the teammate to see. Then their message is emailed out individually to all of the participants of the conversation.	N+u addresses are no longer used in the ReplyTo header. Instead, when a teammate replies in a group conversation their message will be sent in a single email to all participants in the conversation, with those participants email addresses in the To and Cc headers. When users reply to those emails they will be replying directly to each of the participants, plus the Intercom email address used to send the message to them (e.g. bob.oneill@examply.intercom-mail.com). That means we no longer send on users’ replies to every participant. Only the recipients who the user specified in their email will receive it.
Bcc’d or ‘unknown’ conversation participants	When a participant in a conversation replies to an email and Bcc’s someone who is not a participant of the conversation, when the Bcc’d person replies we will split out their reply into a new conversation. There are other similar scenarios, e.g. when a teammate uses multiple email addresses in their client, they may start a conversation from one address then switch to using another. When they do this we would also split out their reply into a new conversation.	We will thread replies from Bcc’d people (or other ‘unknown’ senders) into the same conversation. Their reply will contain a warning banner to tell the teammate that the person wasn’t previously seen in the conversation, and they were not CC’d by another participant. The sender will be added as a participant into the conversation. The teammate can choose to remove them from the conversation if they don’t want that person to see their replies.
Group conversations with one participant on Messenger	Messenger conversations can also become group conversations. Only one of the participants can access the conversation in the Messenger; all other participants only receive replies by email. When the Messenger teammate sends a reply through the Messenger, all participants will receive it. For customers replying, they will reply to the n+u address which means their reply will be added to the Messenger for the Messenger user to see, and forwarded on to the other email participants.	It’s still the case that Messenger conversations can also become group conversations, and that only one of the participants can access the conversation in the Messenger; all other participants only receive replies by email. It’s also still the case that when the Messenger user replies, all participants will get the message, but the email will look like this: sam.murray.onchat@examply.intercom-mail.com to represent the Messenger teammate better. The email will also contain a call-out at the top to inform participants that replying to the email will send it to the user who is on the Messenger.
Conversation history	Customers have a setting in their Email settings to determine whether or not the full history of a conversation should be appended to the bottom of each email sent in a conversation.	Sending conversation history in emails can be enabled or disabled.
User replies from the API	The API can be used to add email replies to conversation on behalf of users. This triggers the reply to be sent to all participants in the conversation.	Behavior remains unchanged.