When a human replies to an email, their client automatically sets In-Reply-To and References headers pointing to the original message. This is what Gmail, Outlook, and Apple Mail use to group messages into a thread. When an AI agent replies to an email incorrectly — missing these headers or generating a new Message-ID — it breaks the thread. The human on the other end sees a new conversation appear out of nowhere, detached from context.

Threading Headers Are Not Optional

The relevant RFCs (2822, 5322) specify that proper threaded replies must include:

  • In-Reply-To: <original-message-id>
  • References: <all-previous-message-ids-in-chain>
  • Message-ID: <new-unique-id>

A naive sendmail call or a simple SMTP library will omit these unless you explicitly set them. Most agent frameworks that build email on top of basic SMTP don't handle this correctly.

How AgentMailr Handles Threading

AgentMailr's reply endpoint manages threading automatically:

// Reply to an email — threading headers set automatically
await client.inboxes.reply("inb_abc123", {
  reply_to_email_id: "eml_xyz789",
  body: "Thank you for reaching out. Here is the information you requested...",
});

The API looks up the original email's Message-ID, constructs the correct In-Reply-To and References chain, and sets a fresh Message-ID for the reply. The thread stays intact in every email client, including the human's inbox.

Thread Context for the Agent

AgentMailr also exposes thread history via the GET /v1/threads/:threadId endpoint, which returns all emails in a conversation ordered chronologically. An agent handling customer support can load the full thread context before composing a reply, ensuring it has the complete conversation history — not just the most recent message.

The Professional Agent Identity

Correct threading signals professionalism. When an AI agent interacts with a human via email, broken threading creates confusion and erodes trust. Infrastructure that handles RFC compliance automatically means your agents present themselves correctly — as competent, reliable communicators — without requiring you to implement email standards yourself.