Google tends to discipline not only the websites but also service providers who used to have many users spamming across the network.
If you are signed up to one of these service providers that google recognizes as spammers this might be the reason why your php mail() messages drops in to spam box in gmail. Try to chat about this issue with your server provider.
In that case you will get a warning from google in your "spam" message:
"Why is this message in Spam? We've found that lots of messages from
home.pl are spam. Learn more"
