Replied-To memo's appear un-read

Recently I have noticed that messages that I have previously read and then replied-to, now appear as though they are un-read. Any ideas?

Subject: Replied-To memo’s appear un-read

Database properties, Design tab (the one with the triangle and T-square), select “Do not mark modified documents as unread”. (The follow-up flag has to modify the mail you replied to in order to show the icon in the view.)

Subject: RE: Replied-To memo’s appear un-read

Thank you it worked perfectly.