Workflow Emails to External Users

I have one SharePoint 2010 server and an Exchange server in my organization. There is also an EXTRANET domain for Clients and Partners.

I would like notifications for a workflow I am building to be able to reach these outside users.

I can see that workflow emails fail for EXTRANET domain people and say "The e-mail message cannot be sent. Make sure the e-mail has a valid recipient"

So, I have had a 'Contact' object created in Exchange and I can manually send an e-mail to this contact and it reaches the outside just fine. However, in SharePoint Designer, I cannot pull up this contact object in the address book in my workflow 'Send an E-mail' step.

How should I go about ultimately reaching this outside contact from a workflow? (FWIW, the tests I am doing is to reach a gmail e-mail address from a workflow)

Thank you!