What is the “References” header?
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The References header is how email clients reconstruct conversation threads. It contains a list of Message-IDs for every email in the conversation so far. The Message-ID of the message being replied to, plus every Message-ID value from that message's own References field. Each new reply appends to the chain.
Email clients use References (along with In-Reply-To) to construct threaded views. Without it, replies that arrive out of sequence, get forwarded, or cross between email clients may appear as disconnected conversations rather than a coherent thread.
For most marketing senders, References isn't something you manage directly. Your ESP handles it automatically. It matters most in two operational contexts: automated reply sequences where you want follow-ups to thread together in a recipient's inbox, and inbound-heavy workflows where you're tracking multi-touch conversations. If your transactional email generates replies you need to correlate, confirm your infrastructure is properly forwarding Message-ID values into the References field on subsequent sends.
And if you're seeing threading problems, replies appearing as new conversations instead of continuations, check that your sending setup is preserving Message-ID values and populating both In-Reply-To and References correctly on outgoing messages.
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