Here is the response I got from GoDaddy:
Thank you for contacting online support. If you use the mail() function in your PHP, you do not need to specify an outgoing mail server. If you are using some other method besides mail() in your PHP code, use relay-hosting.secureserver.net for your relay server.
I assume they are talking about this line in the WA_globals.php file:
$WAGLOBAL_Email_Server = "" ;
I have tried leaving it blank, and using relay-hosting.secureserver.net/ but neither one works.