What is message threading?
It is getting the email client to view messages as threads by including the message reply history with every newly sent message.
The most common use cases for threading are support boxes where you want to keep the message history included in each reply for your recipient’s reference, or messages you send where you expect a reply and a conversation.
How do you thread messages?
The emailing threading depends on the email client used, so it can’t really be guaranteed 100% of the time for all email clients, however, we’ve seen many email clients follow what Gmail does, they require that an incoming message’s Reference header, if present, must reference IDs of previous messages in order to thread (see image below for example). This means that if you receive two emails with the same subject from the same sender, these emails will not be threaded together unless one explicitly references the other.
So we can say the requirements are:
- A reference header with the same IDs as a previous message
- The same recipients, senders, or subject as a previous message
- Sent within one week of a previous message
How do you make sure messages aren’t threaded?
- Send each message with a unique reference header value that doesn’t match a previous message.
- Create a new subject for each message that you don’t want to group.
Message one has this header field:
Message two has this header field: