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Yes, that's the In-Reply-To header. It contains the unique message identifier from the Message-ID field of the email you're replying to.

See also https://tools.ietf.org/html/rfc822 sections 4.6.1 and 4.6.2

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And that's what mature email clients use to present thread trees, to help make sense out of email conversations. Gmail, etc., are not mature email clients.

Observe, furthermore, that References: was in part designed so that a MUA/NUA could reconstruct a tree even if some intermediate messages were missing.

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