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