So you’ve got a site which is supposed to send mails via php mail. But all of your mails end up as spam.
Your reverse DNS-address is set correct, you passed all the necessary stuff to the header but somehow it’s not recognizing the correct return-path you set.
Just pass the return-path as a fifth argument like this:
mail('firstname.lastname@example.org','your subject','your message', 'header', '-f email@example.com');
That’s all, folks(:
But it might be a better idea to use a properly set up mailserver via smtp… much better:)