Email threading

What it means

Threading by the standards follows the Message-ID chain: each reply's In-Reply-To and References point to the messages before it (RFC 5322). Mail programs combine that with other signals. Gmail groups replies into conversations, starts a new conversation when the subject changes or a conversation passes 100 messages, and groups automated mail that shares recipients, senders or subject and reference IDs when sent within a week.

Senders can use this deliberately. Follow-ups that keep the subject and references stay with the first email, which gives the reader context, while notifications meant to stand alone need a unique subject or reference value, which is what Google recommends to prevent unwanted grouping.