I have a CentOS 5.4 box, running ISPConfig 3. I have about 5 additional IPs configured on this server. I have all the IPs configured in ISPConfig (not that it applies to the problem at hand) and have 2 sites setup through ISPConfig. domain1.com and domain2.com domain1.com hosts the nameservers for domain2.com domain1.com points to the primary IP - lets call it 220.127.116.11 domain2.com points to a secondary IP - lets call it 18.104.22.168 When I use PHP mail() to send an e-mail from domain2.com, my "Received From:" header shows the primary IP (22.214.171.124) instead of the IP assigned to domain2.com (126.96.36.199) My question is, how do I get postfix to sign the outbound e-mails from sendmail.postfix as the triggering domain? (In this instance, domain2.com or 188.8.131.52) I know that I can modify ALL outbound headers with /etc/postfix/main.cf and changing $myhostname, but I want the header to be marked with the triggering domain's assigned IP or hostname (either would suffice).