@StefanGries Ok, cool. +1 for the feedback. Interesting input! (maybe for a blog post about alternative mail clients for the mobile..?)
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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.
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More information about Selinux can be found here: http://fedora.redhat.com/docs/selinux-faq/