Reply-To
The email header that sends a reader's reply to a different address while leaving the visible From address unchanged.
See it
What it is
Reply-To is the header that tells a mail client where the Reply button should address the response. Without it, a reply normally goes to the address in From. With it, a receipt can come from 'billing@example.com' while answers go to 'support@example.com', without changing the sender name the reader sees.
Use it when the visible sending identity and the team handling replies are different: support queues, campaign responses, marketplace conversations, or a reply-by-email workflow with a unique address per thread. Keep the destination monitored and make the change obvious enough that a human is not surprised after pressing Reply.
People confuse it with delivery plumbing. Reply-To does not receive bounces, affect SPF or DMARC alignment, or redirect the original message. Return-Path handles bounces and From remains the identity being authenticated. Mail clients are also free to expose or ignore the suggestion, so it is routing metadata, not access control.
Ask AI for it
Update this Nodemailer message so it keeps 'From: Billing <billing@example.com>' but routes human responses to support@example.com using the real 'replyTo' message property. Show the resulting RFC 5322 From and Reply-To headers, add a test that parses the generated message and asserts both addresses, and leave the SMTP envelope sender unchanged so bounces still follow the configured Return-Path. Do not put support@example.com in CC or rewrite From, and call out that Reply-To changes the Reply button target but has no effect on SPF, DKIM, or DMARC alignment.