It has nothing to do with Reply-To=, it indicates that someone has suddenly started using a mail client that uses "> " as a quoting string (instead of, say, ">") and also quotes the headers from the message being replied to in the body of the reply. The solution is for users either to change the quoting string so that it does not include a trailing space or to remove headers from the body of their replies. To disable this part of the loop checking heuristic would practically guarantee loops from mail server that send bounces back in just this way, so it's not recommended. Nathan