imap_mail and outgoing mailserver configurations

hi, i have a problem with imap_mail. the mails i always send with this function contents in the header field 'from' always the server where php is installed. in our system php and the mail server are installed on different machines. so i think that might be the problem. but we configured in the php.ini the outgoing mailserver (smtp section of course). Does anybody could help us to fix the problem to use the right outgoing mail server???