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