Odd situation.
Presumably, if your client has a hosted account with monster.com, that would include email forwarding. If you set up an email address in that account that forwards to the yahoo email address, you should be able to use that address as the 'send to' address, and use monster.com's smtp server settings in your globals.
You could also try (you probably already have) to leave the smtp server settings field blank ( $WAGLOBAL_Email_Server = "" ; ) and then retry sending to both the forwarding address and to the yahoo address (while you're waiting for the techs to come back after the weekend).