Problem "return-path" in email, yahoo and other provider refuse email.
this happened when i changed to a new host with postfix.
To overcome this issue i use the next hack in includes/functions_messenger.php:

$result = $config['email_function_name']($to, mail_encode($subject, ''), wordwrap(utf8_wordwrap($msg), 997, "\n", true), $headers, "-f" . $config['board_email']);