webmin - send mail to remote domain

Hi All,

I got 2 server. First server got installed centos. It also install virtualmin + webmin. I also did simple php script which want to send mail to remote domain aaa.com (second server). But this domain aaa.com is actually one virtual server in first server. We can regard it as local domain. After tried php mail functon, the mail have failed to be sent to correct remote domain (second server). Error is found from mail log.

Jun 3 12:50:32 localhost postfix/error[12237]: D8CD812652C: to=sales@aaa.com, relay=none, delay=0.07, delays=0.06/0/0/0.01, dsn=5.0.0, status=bounced (User unknown in virtual alias table)

After researched info, someone said need to use sendmail server to send mail to remote domain. I don’t know how to use it from webmin. Please advise me what do i should to do.