simple question, why emails wich i send it with php mail function using postfix on linux, goes in spam folder? Headers seems to be ok. I also try without headers. So, how to solve a problem?

Goes in spam folder of what email client? see what your filter settings are.

My site also did this when I got emails from the php I use. Tried to add the address to the safe list but it didn't work.

The problem is, when users register on my site and if their email provider doesn't had a spam folder.