I have it working. It did take a little tinkering to get the path set correctly in my cpanel, but overall that feature currently works fine on my site,
And Ben is correct with all of his instructions.
However, based on your first question, the purpose of the feature is not so users can compose a blank email message in any email client to send to any user. The first message must come from within the FEP messaging portal on your site. When the recipient receives an email notification that there is a message waiting for them on your site, that’s the email they can reply to, to send a message back to the sender.
Hope that helps, and good luck.