sendmail_from in linux installation

Hi All: We are serving some virtual domains at dsland.org and found with a problem concerning the limitation of to prevent/track mail sent by our users. I would like to include sendmail_from directive in the apache's virtual server configuration in order to fix the from field in mail sent by our sendmail server via php and perharps to trace in system logs. I think must be a reason for use sendmail_from directive only in Windows but I can't imagine that. I've solve this problem by recoding the php's mail function and replacing/adding any From entry which the sendmail_from but I think must be a reason. Do anybody of you know which is the reason for this limitation? Sorry for my very bad English. I would appreciate replies to spam@dsland.org.