> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agentpost.email/llms.txt
> Use this file to discover all available pages before exploring further.

# Reply to Email

> Reply to a received email from a custom domain mailbox.

When you reply to an inbound message, AgentPost pre-fills the compose dialog and threads the reply correctly using standard email headers. Like composing from scratch, replies require a custom domain mailbox to send from.

## Reply flow

<Steps>
  <Step title="Open a received message">
    Navigate to any inbound message in your mailbox.
  </Step>

  <Step title="Click Reply">
    Click the **Reply** button. The compose dialog opens pre-filled with:

    * **To** — the original sender's email address
    * **Subject** — the original subject prefixed with `Re:`
    * **In-Reply-To** — the Message-ID of the original message (set automatically for email client threading)
  </Step>

  <Step title="Select a From address">
    Choose which custom domain mailbox to reply from using the **From** dropdown. Only mailboxes on domains that can currently send (status `active` or `pending_warmup`) are listed.
  </Step>

  <Step title="Edit the recipients or subject (optional)">
    Adjust the **To** field, or expand **CC / BCC** to add copied recipients. Edit the subject if needed.
  </Step>

  <Step title="Write your reply">
    Enter your message in the body field.
  </Step>

  <Step title="Attach files (optional)">
    Select files using the attachment button. Files upload immediately and are held as drafts until you send. The same limits apply as when composing: 25 MB per file, 25 MB total per message.
  </Step>

  <Step title="Send">
    Click **Send**. AgentPost creates an outbound `Message` record linked to the original conversation thread, then delivers the reply.
  </Step>
</Steps>

## Threading

Replies use the standard RFC `In-Reply-To` email header so the recipient's email client displays the conversation in thread order. AgentPost groups the reply into the same conversation thread as the original message, keeping the full back-and-forth visible in your mailbox.