PHP mail function is sending emails in Gmail's SPAM folder, How to get rid of it?

i am using this code :

$to = "someone@example.com";
$subject = "Test mail";
$message = "Hello! This is a simple email message.";
$from = "someonelse@example.com";
$headers = "From:" . $from;
mail($to,$subject,$message,$headers);
echo "Mail Sent.";

it sends email to my gmail's SPAM folder .

Suggest any solution .

i dont want to use any PEAR MAIL type way to send email or dont want to require and include any file.

This functions works without including/requiring any additional php file.

You should use proper header like this from PHP manual

$headers  = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";

// Additional headers
$headers .= 'To: Mary <mary@example.com>, Kelly <kelly@example.com>' . "\r\n";
$headers .= 'From: Birthday Reminder <birthday@example.com>' . "\r\n";
$headers .= 'Cc: birthdayarchive@example.com' . "\r\n";
$headers .= 'Bcc: birthdaycheck@example.com' . "\r\n";

// Mail it
mail($to, $subject, $message, $headers);

The Gmail spam filter has lots of parts to it which determine the spamminess of mail. You could start by seeing the SPF record for your domain. Avoid using sales language, HTML and colours where possible in your email.

There's also an old but fun video from Google about how their spam filtering works.

It's all in the headers. I don't know how GMail works internally, but I've found on my projects that setting Reply-To can Content-Type headers seems to fix this.

See PHP mail() docs for an example of doing this.

(Note: I've got a PHP questionnaire/response system that sets these headers and the e-mails come through correctly to GMail, Hotmail and Yahoo! Mail).