Spam and how to make warning with php in website when there is an new email

PHP
1

i use mail() to send an email but it always sent in spam, how to fix it? and i want to make php script to show something like “warning” in my website when there is any new email, how to do it?
please, really need your help!

2

Spam mails is a complex subject.

First off, are you sending a properly formatted header? The fourth parameter in mail();

Besides that, the rest of the settings are outside of PHP, namely on the mail server.