The certificate for the site mail.xfce.org expired on 2015-07-18. As a consequence it is inaccessible with a properly configured browser.
same again for certificate:
mail.xfce.org uses an invalid security certificate. The certificate expired on 10/03/2016 12:10 PM. The current time is 10/04/2016 04:04 PM. Error code: SEC_ERROR_EXPIRED_CERTIFICATE
We now use let's encrypt, this problem is gone.