Hello, If a message contains the "Reply-To" header, the functionality "Reply to author |Shift+A]" will not work as expected since it will always reply to the email(s) listed in "Reply-To" header instead of replying to the person who sent the message "From" header. Best regards, Wassime.
KMail is working as intended. See the standard, https://tools.ietf.org/html/rfc822: "Reply-to" overrides "From", with the assumption that authors deliberately set it because they want replies to go to a particular address.