IMP 6.2.

Changes have been made in Git (master):

commit 64d063960e79687bf59925599a17f7ce63e35bbd
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Mar 31 23:03:09 2014 -0600

     [mms] Display flags on the smartmobile mailbox page (Request #12687).

  imp/docs/CHANGES                                 |    1 +
  imp/js/smartmobile.js                            |   60 
++++++++++++++++++----
  imp/lib/Ajax/Application/Handler/Dynamic.php     |    4 +-
  imp/lib/Ajax/Application/Handler/Smartmobile.php |   17 ++++++
  imp/lib/Ajax/Queue.php                           |   51 +++++++++++-------
  imp/package.xml                                  |    1 +
  imp/themes/default/smartmobile/screen.css        |    6 ++
  7 files changed, 108 insertions(+), 32 deletions(-)

http://github.com/horde/horde/commit/64d063960e79687bf59925599a17f7ce63e35bbd

Not very useful.  Having a message in italics does not indicate to me 
that a message is answered, because that's not consistent in what we 
do in the other views.  So that's not a viable solution.

I'm open to adding further information to the mailbox screen, but this 
won't be changed until IMP 6.2 at the earliest, so this is a low 
priority right now.

In Smartmobile mode, answered or forwarded emails can't be 
distinguished from Seen/Unseen emails.

In Basic/Dynamic modes, answered/forwarded emails are displayed with 
different background color and with arrow icons.

A simple solution could be to display answered/forwarded emails with a 
different font style.
For example, the following code displays the subject of answered 
emails in italics:

--- imp/themes/default/smartmobile/screen.css.orig      Wed Jul 17 
09:36:40 2013
+++ imp/themes/default/smartmobile/screen.css   Wed Sep 11 16:39:37 2013
@@ -30,6 +30,10 @@
  .imp-mailbox-seen a {
      font-weight: normal;
  }
+.imp-mailbox-answered a {
+    font-weight: normal;
+    font-style:italic;
+}

  .divSwipe a {
      font-weight: bold;

--- imp/js/smartmobile.js.orig  Wed Jul 17 09:36:33 2013
+++ imp/js/smartmobile.js       Wed Sep 11 16:38:12 2013
@@ -553,6 +553,10 @@
                      case IMP.conf.flags.seen:
                          c.addClass('imp-mailbox-seen');
                          break;
+
+                    case IMP.conf.flags.answered:
+                        c.addClass('imp-mailbox-answered');
+                        break;
                      }
                  });
              }

--- imp/lib/Smartmobile.php.orig        Wed Jul 17 09:36:34 2013
+++ imp/lib/Smartmobile.php     Wed Sep 11 16:40:43 2013
@@ -161,6 +161,7 @@
                  'flags' => array(
                      'deleted' => '\\deleted',
                      'draft' => '\\draft',
+                    'answered' => '\\answered',
                      'seen' => '\\seen'
                  ),
                  'mailbox_return' => $prefs->getValue('mailbox_return'),