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?