Back to Tools

Email Thread Explainer

Understand how email threading works — from Re: and Fwd: prefixes to the Message-ID, In-Reply-To, and References headers that connect conversations. Learn how Gmail, Outlook, Apple Mail, and other clients thread emails differently, why marketing emails shouldn't thread, and best practices for managing threads in professional email.

About this tool

Why email threading matters

Email threading is one of those things most people never think about until it goes wrong. You send a reply and it doesn't appear in the conversation. Or an unrelated email gets merged into a thread it doesn't belong in. Understanding how threading works helps you communicate more effectively and avoid these frustrating situations.

For email marketers, threading has specific implications for deliverability and engagement. Marketing campaigns that accidentally thread can bury your new message inside an old conversation, dramatically reducing visibility.

The two mechanisms behind threading

Email clients use two approaches to group messages into threads:

  • Header-based threading: Using Message-ID, In-Reply-To, and References headers to create explicit parent-child relationships between messages. This is the RFC-standard approach and the most reliable.
  • Subject-based threading: Grouping messages with the same subject line (after stripping Re:/Fwd: prefixes). Gmail famously uses this approach, which is why it sometimes merges unrelated emails.

Related tools

Frequently Asked Questions

What does Re: mean in email?

Re: is a prefix automatically added to the subject line when you reply to an email. It tells the recipient (and their email client) that this message is a response to a previous one. The origin is debated — it either comes from the Latin 're' meaning 'in the matter of' or is simply short for 'reply.' Either way, it's a visual indicator that a message belongs to an existing conversation. Most modern email clients collapse multiple Re: prefixes (Re: Re: Re:) into a single Re: for cleanliness.

What's the difference between Re: and Fwd:?

Re: means you're replying to someone who was already in the conversation — your response stays in the same thread. Fwd: means you're sending the email to someone new who wasn't in the original conversation. Importantly, forwarding breaks the thread — the new recipient sees it as a standalone message, not part of the original thread. Outlook uses RE: and FW: (uppercase, no 'd') while most other clients use Re: and Fwd: — the difference is purely cosmetic.

Why does Gmail merge unrelated emails?

Gmail threads primarily by subject line. If two unrelated emails happen to have the same subject (like 'Meeting Tomorrow'), Gmail may group them into one conversation. This is by design — Google prioritizes fewer, consolidated threads. The only workaround is to use specific, unique subject lines. You can also turn off conversation view in Gmail Settings → General → Conversation View → Conversation view off, though this disables all threading.

How do email headers control threading?

Three headers control email threading: Message-ID (a unique identifier for every email), In-Reply-To (contains the Message-ID of the email being replied to), and References (lists all Message-IDs in the conversation chain). When you reply, your email client sets In-Reply-To to the parent message's ID and adds all ancestor IDs to References. The receiving client uses these to reconstruct the conversation tree. Subject-line matching is a secondary, less reliable mechanism.

Should marketing emails be threaded?

Almost never. Each marketing campaign should be a standalone message. If your campaigns accidentally thread (because of reused subject lines or reply-chain headers), your new campaign gets buried inside an old thread — most recipients won't even see it. Legitimate cases for threading: transactional sequences (order → shipping → delivery) and 1-to-1 sales follow-ups that genuinely continue a conversation. Good ESPs like Sequenzy automatically prevent marketing campaign threading by using unique Message-IDs and omitting reply headers.

Is it okay to add Re: to cold emails?

No. Manually adding 'Re:' to a cold email subject line to imply an existing conversation is deceptive. It violates CAN-SPAM (misleading headers), damages trust with recipients, and increases spam complaints. While it might boost short-term open rates, the long-term cost to your sender reputation and brand isn't worth it. Be direct and honest in your subject lines — cold email works best when you offer genuine value, not when you trick people into opening.

How do I break out of an email thread?

To start a new topic without being stuck in an old thread, compose a new email (don't reply to an existing one). Even if you change the subject in a reply, the headers still link it to the old thread — most email clients will still group it with the original conversation. If you need to reference the old thread, you can include 'Continuing from the budget discussion...' in the body of your fresh email for context.

What does the Thread-Index header do?

Thread-Index is a Microsoft-specific header used by Outlook for threading. It contains a Base64-encoded value that tracks the conversation hierarchy, including timestamps and nesting levels. When both sender and recipient use Outlook, Thread-Index provides precise threading. Non-Microsoft email clients ignore this header entirely and rely on the standard In-Reply-To and References headers instead. This is why threads can look different in Outlook vs. Gmail or Apple Mail.

Compare email marketing software

Hands-on roundups to help you pick the right platform.

More Free Tools

View all tools

Email Sign-Off Generator

Browse 40+ professional email sign-offs organized by tone and context. Filter by professional, friendly, casual, grateful, or creative. Includes situation-specific recommendations and a random picker for when you're stuck.

Professional Email Phrase Generator

Find better alternatives to overused email phrases. Replaces 'just checking in,' 'per my last email,' 'friendly reminder,' and 12+ other cliches with professional, friendly, casual, or direct alternatives.

Email Closing Generator

Browse 35+ professional email closing sentences organized by tone — professional, friendly, casual, urgent, grateful, and encouraging. Includes copy-paste closings, formulas by email type, and tips for crafting the perfect ending.

Email Marketing Abbreviation Reference

A comprehensive glossary of 70+ email marketing abbreviations, acronyms, and technical terms. Search by keyword or filter by category — from metrics (CTR, CTOR, RPE) to authentication (SPF, DKIM, DMARC) to compliance (GDPR, CAN-SPAM, CASL). Each entry includes a plain-English definition, real-world examples, and links to related terms.

Related reading