Most conversational state management assumes the conversation is happening — a chat session, a websocket, a context window. Email breaks that assumption rudely: a customer replies five days after your agent's last message, and your code is expected to pick up exactly where things left off, with no session, no socket, and a process that has restarted twelve times since.
The good news: email already solved durable conversation tracking, decades ago, in three headers. Build on them properly and the thread itself becomes the agent's memory. This is the pattern behind the multi-day support agent recipe, which runs an LLM support agent on its own mailbox via Agent Accounts — currently in beta.
The three headers doing the work
Every email carries a globally unique Message-ID. A reply adds In-Reply-To (the Message-ID being answered) and References (the full chain of Message-IDs, oldest to newest). That's how Gmail, Outlook, and Apple Mail all decide what belongs to one thread — subject-line matching is only a fallback, and the email threading docs explain why relying on it breaks: recipients edit subjects, two prospects can receive identical subjects, and forwards keep the subject while changing the conversation entirely.
