I am using RHEL4 as a web server, using sendmail as the default SMTP mail to send out email form a PHP application. The Sendmail relays to my company's mail server(domino), What I did was to set the smarthost as the domino mail server in the Sendmail configuration file. It works. But now domino mail server has some unrouted out mail says like: SMTP Protocol Returned a Permanent Error 553 5.1.8 <apache@localhost.localdomain>... Domain of sender address apache@localhost.localdomain does not exist How can I make the sender address apache@localhost.localdomain appeard as a valid sender address? In the mail log, the ctladdr=<apache@localhost.localdomain> Thanks for your help! CZ