What headers are preserved when replying?

So when When you hit reply, your email client preserves three things from the original message: the In-Reply-To header (which points to the Message-ID of the email you're replying to), the References header (which tracks the entire conversation thread), and the Reply-To address if the sender set one.

Everything else gets rebuilt from scratch. The Received headers (the routing stamps showing which mail servers handled the original), authentication results (SPF, DKIM, DMARC checks), and Return-Path are all generated fresh for your reply. Your reply isn't piggybacking on the original's delivery path. It's a brand new message that happens to know which conversation it belongs to.

The Reply-To header is worth understanding if you send email professionally. When you set a Reply-To address, you're telling the recipient's email client "when they hit reply, send their response here instead of the From address." This is common for newsletters sent from noreply@example.com but Reply-To set to hello@example.com, or for support tickets where the From is tickets@company.com but replies should go to the customer's actual support agent.

Why this matters for senders: if you want replies to land somewhere specific, set Reply-To explicitly. Don't assume recipients will notice a different address in your signature. Email clients respect Reply-To automatically, so it just works. (Though some senders misuse it to make their From look clean while hiding their real sending address, which can feel shady to recipients.)

Why this matters for troubleshooting: if someone replies to your email and you never get it, check whether you set a Reply-To that points to the wrong mailbox. We've seen this a dozen times. The reply went exactly where you told it to go, just not where you thought you told it to go.

Want to see which headers your own emails carry? Paste one into our Email Header Analyzer and it'll show you In-Reply-To, References, Reply-To, and everything else in plain English.