Reporting mail as spam

Hi, I wrote a little registration routine, which will send a confirmation letter to the user with a random number in the message body (my site is on a host, so I can't write in the subject, and ask the user to reply), which can be clicked then, and my site will finish the registration. My big problem is, that this host inserts an X header to the mail which identifies my PHP script as the X-PHP. As I recognize, this header adds a huge number to the spam score. Is there any possibility, to reduce the other scores? By the way, what counts most in a spam? Thanks, SanTa