I have installed the smtp server and Xampp

Warning: mail() [function.mail]: SMTP server response: 550 Invalid recipient: xxxx@yahoo.com in C:\xampp\htdocs\emailtesting.php on line 4

<?php

ini_set("sendmail_from","xxxx@yahoo.com"); 
mail("xxxx@yahoo.com","Success","Thanks", "From: yyyy@yahoo.com");

?>

Try removing the default sender (sendmail_from) in your php.ini. That has helped my friends in the past.

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