We are going to start a closed BETA for the S/MIME plugin this week. Send us a DM when you want to join (not a lot effort required)!
When SELinux is enabled, this is blocking your connection from the webserver to the Zarafa server.
You may solve this by allowing Apache to make network connections:
or by disabling SELinux altogether:
When you choose to disable SELinux, you will also want to edit /etc/sysconfig/selinux to disable it for reboots too.
Hints en Tips
More information about Selinux can be found here: http://fedora.redhat.com/docs/selinux-faq/