How to group email replies into threaded conversations (Gmail)
Most email software programs (including Gmail) give the user an option for new Inbox messages.
New email can show up in your Inbox separately or be grouped into a "conversation". This means all emails from the same topic will be combined and shown together.
How does a conversation look in Gmail?
A grouped conversation presents as just one line in the Inbox. When this line is clicked, it opens to show all the messages in that conversation thread.
How does Gmail decide if an email should be in a "conversation"?
In general, if the email is a Reply to the same recipients, senders, or subject as a previous message, then it will be shown grouped in a conversation.
Technically speaking, Gmail is also looking for the In-Reply-To and References fields that are hidden in the email headers.
How do I change this in Gmail?
For help, go to Group emails into conversations.