The list of available plugins is at:

l /var/www/imscp/gui/public/tools/webmail/plugins/ | awk '{print $9;}'

acl
additional_message_headers
archive
autologon
database_attachments
debug_logger
emoticons
enigma
example_addressbook
filesystem_attachments
help
hide_blockquote
http_authentication
imscp_pw_changer
jqueryui
managesieve
markasjunk
new_user_dialog
new_user_identity
newmail_notifier
password
redundant_attachments
serverinfo
show_additional_headers
squirrelmail_usercopy
subscriptions_option
userinfo
vcard_attachments
virtuser_file
virtuser_query

Plugins are activated if the name of their directory is added to an array in the roundcube configuration file located at /var/www/imscp/gui/public/tools/webmail/config/main.inc.php

By default, i-mscp activates only one of those plugins: imscp_pw_changer

To activate the plugins additional_message_headers and archive, I would change the line:

$rcmail_config[‘plugins’] = array(‘imscp_pw_changer’);

To:

$rcmail_config[‘plugins’] = array(‘imscp_pw_changer’,’additional_message_headers’,’archive’);

To get the linenumber of this line, just type:

grep -in 'plugins' /var/www/imscp/gui/public/tools/webmail/config/main.inc.php

Just restart apache2 and refresh your browser and you will see that the plugin has been activated. The complete list of plugins can be found at http://trac.roundcube.net/wiki/Plugin_Repository

To add a new plugin, extract the downloaded archive to its own directory at /var/www/imscp/gui/public/tools/webmail/plugins and add its name to the array list in /var/www/imscp/gui/public/tools/webmail/config/main.inc.php