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.
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Frequently Asked Questions
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