Php mails going in spam

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PHP

my php mails ie confirmation mail always go in spam folder, but when i mark hat mail as NOT SPAM, php confirmation mail never comes in inbox or in spam. What to do, please help

What I have tried:

$to = $email;
$subject = "Verify your account ";

$body = "Dear $name



$header = "From:admin@xyz.com \r\n";
//$header .= "Cc:afgh@somedomain.com \r\n";
$header .= "MIME-Version: 1.0\r\n";
$header .= "Content-type: text/html\r\n";

$retval = mail ($to,$subject,$body,$header);

if( $retval == true ) {
header("location:verify.php");
}else {
echo "Message could not be sent...";
}

//echo "

Details Saved!

";
}
}
else
{
echo 'Error' . mysqli_error($conn);
}
}