sending emails

We have web forms that send the user an email confirmation after submission, like most forms do. The emails are being delivered to the users' junk folder. The main campus IT staff claim it is because our server is sending the emails. The campus is using Microsoft exchange servers. I am using Red Hat Linux, sendmail, and PHP. Is there a way to give php the exchange server's ip address and have the emails from my php forms be sent from the exchange server? Thanks Marc