In each of the security role, there's an option under the business management tab to allow you to enable "Send Email as Another User" for regular email activities.
Update the security role, then try again to see if you will able to send it as another user.Darren Liu | 刘嘉鸿 | MS CRM MVP | English Blog: http://msdynamicscrm-e.blogspot.com | Chinese Blog: http://liudarren.spaces.live.com