[PHP] sending mail via php and unix box-clients thinks it's spam

[blackwater dev]

I have a site running on a linux box.  If I use my mail server via an email
client, the email works fine but if I send email through the mail function,
clients mark it as spam.  I see the line in php.ini for SMTP but that says
windows only.  How do I tell php to use my mail server?

Thanks!