What is the “In-Reply-To” header?
The In-Reply-To header links a message to a specific previous email by referencing that email’s Message-ID. Think of it as the rope tying two ships together in conversation. It allows mail clients like Gmail.com and Outlook.com to organize replies into proper threads instead of scattering them across the inbox.
When replying to an email, the In-Reply-To header shows the unique ID of the message you responded to. Without it, threads can break apart, confusing both users and systems.
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