replyto

ReplyTo is an email header field that specifies the address or addresses to which replies should be directed. It can differ from the From address, allowing the sender to direct responses to a different mailbox, department, or distribution list. For example, a message might be sent from [email protected] with a Reply-To: [email protected] header so replies go to the support team.

In practice, email clients use the Reply-To address when the user chooses to reply. If the Reply-To field is absent, the reply is typically addressed to the From address. The field can contain a single mailbox or multiple addresses, and it may include display names. Its syntax follows the standard email header format, such as Reply-To: "Customer Support" <[email protected]>, with multiple addresses separated by commas as needed.

Usage considerations include its role in mailing lists and customer support workflows. Lists and automated systems often set or rewrite the Reply-To header to ensure responses reach the intended group. However, because the header can be specified by the sender, it can also be misused to direct replies to an unintended address, which has security and trust implications.

Relation to other headers: Reply-To is distinct from From (the purported author of the message) and To/CC (the recipients of the message). It is also separate from In-Reply-To, which identifies the message being replied to for threading, and from Return-Path, which indicates the address for bounce messages. Overall, Reply-To provides flexible control over where replies are delivered.