What Is Email Threading? Headers, Replies and Agent Context

Email threading groups related messages into a conversation. For an agent, that means a reply can be interpreted alongside the request and previous responses instead of as a new, unrelated task.

This guide explains the email headers behind a thread, the identifiers your application needs to keep distinct and how to debug a conversation that splits. For the complete sending and receiving interface, see the email API for agents.

The three email headers that connect a conversation

Message-ID identifies an email message at the protocol level. It commonly looks like <unique-value@example.com>. Keep it separate from an API's database ID for the same message.

In-Reply-To identifies the earlier email being answered. A reply to <original@example.com> should reference that value rather than the database UUID used to retrieve the original message.

References records the chain of earlier message identifiers. Mail clients and servers can use these headers to associate messages with a conversation.

Subjects can be useful fallback signals, but “Re: Support request” is not a unique identifier. Two unrelated customers can send the same subject. Never use subject matching as a security boundary or a reliable substitute for a stored conversation mapping.

Three identifiers your application should keep distinct

Identifier	Use
Robotomail message ID	Fetch a specific message through the API; deduplicate application processing
RFC email Message-ID	Set the reply relationship in inReplyTo
Robotomail thread ID	Retrieve related messages for conversation context

Use the response fields in the messages reference rather than guessing which ID a webhook example contains. Fetch the original message if an event does not contain the field your next step needs.

Reply from the same workflow

After receiving and validating a message, your application can send a reply:

curl -X POST https://api.robotomail.com/v1/mailboxes/MAILBOX_ID/messages \
  -H "Authorization: Bearer $ROBOTOMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "to": ["customer@example.com"],
    "subject": "Re: Your request",
    "bodyText": "Thanks for the detail. Here is the next step.",
    "inReplyTo": "<original-message-id@example.com>"
  }'

Replace the mailbox ID, recipient and original RFC Message-ID with values validated by your application. A quoted email address or a model suggestion is not authority to send private information to that address.

Load the relevant thread before generating a response. Include enough context for the task, while excluding unrelated messages and data the agent is not allowed to access. Save the action outcome outside the model so a restarted worker knows whether it already replied.

Why email clients sometimes group messages differently

Different clients can apply different combinations of reply headers, subject normalization, participants and other signals. A message appearing in one local thread does not guarantee every recipient's client will display it identically.

Forwarding is particularly important: a forwarded message may start a new conversation even when its body quotes an older one. A changed subject can also change the visible grouping. Do not treat quoted text as a trustworthy header or let it override your application's ticket mapping.

For shared mailboxes and multi-agent workflows, explicitly store which business task owns each thread. A thread can contain several requests, and a request can span several threads. Your application owns that business relationship.

Debug a broken email thread

1. Retrieve the sent message and the received reply. Compare their RFC Message-ID, In-Reply-To and References values.
2. Confirm that inReplyTo contains the original email identifier, not a database UUID or a thread ID.
3. Check the sender mailbox and intended recipients. Avoid generating a new sender identity unintentionally.
4. Inspect the full message in representative recipient clients, including any forwarding or subject changes.
5. Check whether the application processed a retry and sent a duplicate response.
6. Keep message IDs and task state in structured logs, with access controls and minimal private content.

See the thread API and receive-and-reply guide for the integration details.

Threading is context, not permission

A reply in an existing thread can still contain a malicious instruction, a new recipient or a request for data the sender should not receive. Apply authorization and action checks independently of conversation grouping.

Use agent email security to set those boundaries. Then connect the complete loop with Robotomail's email API.