As recommended at https://cr.yp.to/proto/replyto.html, give the
highest priority to the Mail-Reply-To header when determining the address
of the author of a message. (See also D24481.)
Also, if the original message was from the KMail user to someone else,
ReplyAll's behaviour is now consistent with ReplySmart: the reply comes
from the user and goes to the original recipients.