just so you know:
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 04:16 PM. Error code: SEC_ERROR_EXPIRED_CERTIFICATE
Looks like its been fixed:
Issued On Tuesday, October 4, 2016 at 3:26:00 PM Expires On Monday, January 2, 2017 at 2:26:00 PM
I'm not sure which developer looks after the certificates, but creating a bug report or posting a message to the #xfce-dev IRC channel should get their attention.
3 months is a rather short certificate.