Project "From" address property not editable

I've attempted to set up the helpdesk configuration as described at http://blog.jetbrains.com/youtrack/2014/02/using-youtrack-as-a-help-desk/

It's generally working as detailed, barring one crucial point which is that I'm unable to set a project-specific "from" address as the field is grayed out and populated with the server "from" address defined in the general settings.



Any suggestion as to why I'm unable to edit this value and how I can resolve it?