yahoo servers are picky about the from address of the email being sent.
here are a couple things to try:
WA_Universal_Email/WAUE_contact_1.php file.
Find the line:
$MailFrom = "".((isset($_POST["Email_address"]))?$_POST["Email_address"]:"") ."";
and change that to an address from your domain:
$MailFrom = "somebody@myDomain.com";
also, in the mail_PHP.php file, try changing:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader );
to:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader , "-f".$mailFrom);