Add 'mailfollowupto' to $rcmail_config['plugins'] in Roundcube's config file config/main.inc.php
View MFT/MRT
You may use the show_additional_headers plugin to let Roundcube show MFT and MRT when viewing messages. It's shipped with Roundcube, you just need to activate it:
Add 'show_additional_headers' to $rcmail_config['plugins'] in Roundcube's config file config/main.inc.php
Add the line $rcmail_config['show_additional_headers'] = array('Mail-Followup-To', 'Mail-Reply-To');
to Roundcube's config file config/main.inc.php