It’s basically how services like Hotmail/Live/Outlook and Gmail alert
people that the email was sent from a mail server that’s not explicitly
authorized to send mail for that domain. Add SPF and DKIM records for the
sending domain (from domain).
Also… I’d use these headers instead of the Sender header:
RT-Attach-Message: yes
Landon Stewart :: lstewart@iweb.com
Lead Specialist, Abuse and Security Management
Spécialiste principal, gestion des abus et sécurité http://iweb.com :: +1 (888) 909-4932