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Overview

A thread (or conversation) is a group of related messages linked together by their reply chain. When you reply to an email, Jasni automatically maintains proper threading so messages appear grouped in the recipient’s email client.

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How Threading Works

Email threading relies on three headers:
HeaderDescription
Message-IDUnique identifier for each email
In-Reply-ToReferences the parent email’s Message-ID
ReferencesFull chain of Message-IDs in the thread
When you use Jasni’s reply or forward endpoints, these headers are automatically set correctly.

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Replying to Messages

Reply to a message with proper threading: 
POST /api/v1/emails/123/[email protected]
{
  "text": "Thanks for your message!",
  "replyAll": false,
  "includeOriginal": true
}
Jasni automatically:
  • Sets the correct In-Reply-To and References headers
  • Determines the correct recipients
  • Adds Re: prefix to the subject
  • Quotes the original message (if includeOriginal is true)

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Reply vs Reply All

OptionBehavior
replyAll: falseReply only to the sender
replyAll: trueReply to sender and all CC recipients

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Forwarding Messages

Forward a message to new recipients: 
POST /api/v1/emails/123/[email protected]
{
  "to": ["[email protected]"],
  "text": "FYI - see below"
}
Forwarded emails:
  • Maintain the original thread context
  • Add Fwd: prefix to the subject
  • Include the original message content

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Thread Structure Example

Thread: "Project Update"
├── Message 1: "Project Update" (from: alice@...)
│   └── Message 2: "Re: Project Update" (from: bob@...)
│       └── Message 3: "Re: Project Update" (from: alice@...)
│           └── Message 4: "Re: Project Update" (from: charlie@...)
Each reply links back to its parent via In-Reply-To, and contains the full chain in References.

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Building Thread Views

To display a conversation view in your application:
  1. Fetch messages from a folder
  2. Group messages by their References or root Message-ID
  3. Sort within each group by date
// Example: Group messages into threads
function groupIntoThreads(messages) {
  const threads = new Map();
  
  for (const message of messages) {
    // Get the root message ID from References or use Message-ID
    const threadId = message.references?.[0] || message.messageId;
    
    if (!threads.has(threadId)) {
      threads.set(threadId, []);
    }
    threads.get(threadId).push(message);
  }
  
  // Sort messages within each thread by date
  for (const thread of threads.values()) {
    thread.sort((a, b) => new Date(a.date) - new Date(b.date));
  }
  
  return threads;
}

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Best Practices

Don’t manually construct reply headers. Use the /reply endpoint to ensure proper threading.
Avoid changing the subject line in replies, as some email clients use the subject for threading.
When replying, set includeOriginal: true to quote the original message for context.

Messages

Learn about messages

Reply API

Reply endpoint

Forward API

Forward endpoint