Currently Reviewboard sends emails with a Return-Path of the current user. This is not SPF compliant, and as a result review emails will be rejected if a user's domain implements SPF with a reject all failed messages policy and a reciever checks SPF.

This corrects this by setting the From header directly instead, and passing the site default from email address as the Return-Path address.
This is needed on the KDE Reviewboard instance to ensure email delivery runs well.

Local development 1.6.x instance. From header is still set correctly, whilst Return-Path is now SPF compliant.