Windows MTA connect to Multiple Gmail accounts

I recently moved from having an internal Exchange server host all my mail to using Google Apps. Everything is fine, except I need to be able to send mail from internal systems (SharePoint workflows, SQL notifications, etc.) using several different Google Apps mail accounts.

I setup an SMTP virtual server and can successfully relay my mail to (and through) Google, but in the SmartHost authentication I have to specify a specific account to login to Google with. When I send mail through this SMTP virtual server, Google automatically rewrites the "from:" address to be the "from:" address of that authenticating account.  

So the question is, is there a third-party MTA out there that will allow me to specify multiple smarthost authentication accounts and choose the one to use based on the originating "from:" address? I know I can setup multiple SMTP virtual servers/MTAs, each with a different authenitcaing account, but I cannot setup multiple outbound gateways on my source servers (SHarePoint, SQL, etc.) to point to the differnet gateways on my network.

Thank you in advance!