PHP has built-in mail() function which can be used to send email without authentication. You just need to make sure that server is properly configured to relay email via PHP script.
When you use PHP mail(), it does not use SMTP authentication to relay email. Following us the sample script to send email using PHP mail() function:
$email = "email@example.com";
$message = "This is a test email using PHP mail() function" ;
// Here is PHP mail() function
mail("firstname.lastname@example.org", "Mail from PHP mail",$message, "From: $email");
echo "Mail Sent";