The "bad security" error is often due to a site using self-signed certificates. If this is the case, explicitly naming the mail host probably will not change anything. If the cause is a self-signed certificate, you could try to access the site with your browser and add an exception for the certificate (as described in the article you listed above). If that does not work, you have to manually add the certificate to your phone following the procedure described in the article you listed above (also shown here: http://www.pending.io/add-cacert-root-certificate-to-firefox-os/). I followed the path described there and was able to get my e-mail to work.