In-Reply-To / References
Email headers (In-Reply-To and References) that link a reply to the message it responds to, enabling mail clients and archive tools to group related messages into conversation threads.
When you reply to an email, your mail client sets two headers in the outgoing message: In-Reply-To contains the Message-ID of the single message you are directly replying to, while References contains the full chain of Message-IDs from the entire conversation up to that point. Together they form the explicit link graph that threading algorithms use to reconstruct conversations.
The JWZ threading algorithm (named after Jamie Zawinski, who described it for Netscape Mail) uses In-Reply-To and References to build a tree of messages. Messages without explicit references can be matched heuristically by Subject (stripping Re: and Fwd: prefixes) as a fallback. The result is a grouped view where all messages in a conversation appear together regardless of the order they arrived.
In-Reply-To and References are critical for navigating large archives. Without thread reconstruction, a mailbox with thousands of messages is a flat chronological list; with threading, related messages collapse into conversations that are much easier to scan. Mbox Viewer reconstructs threads from these headers when displaying MBOX archives.
Related terms
A globally unique identifier assigned to each email message, specified in the Message-ID header. It is used to track messages, build conversation threads, and detect duplicates when merging archives.
The process of grouping related email messages into conversations by following In-Reply-To and References header links, typically using the JWZ algorithm that supports up to four levels of nesting.