How does Zendesk handle Reply-To?

Question

How does the email server of Zendesk handle a Reply-To: in email headers? 

Answer

The mail servers of Zendesk use the Reply-To: line in the headers, if present, to identify the author of the email. Since the Reply-To: line in the header indicates the mailbox to which responses should be sent, this field supersedes any data in the "From:" field.

For more information on how incoming emails thread in tickets, see the article: Why do emails thread to the wrong ticket?

Note: This Reply-To: header is an official email standard and Zendesk must respect that email standard: "The originator fields also provide the information required when replying to a message. When the "Reply-To:" field is present, it indicates the address(es) to which the author of the message suggests that replies be sent. In the absence of the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field unless otherwise specified by the person composing the reply."

Zendesk will use the Reply-To: as the originator/sender in keeping with this RFC standard. If From: and Reply-To: differ, Zendesk uses the Reply-To:. This can cause unexpected behavior with emails if the sending server mishandles these headers.