Autopopulating Field with Realted Data via email input. (got that? lol)

I thought for 10 min and could not come up with a more descriptive subject. ;-)

I have a form that is populated via email from multiple websites. I have a "source" field to track which site sent the mail. (So if the form was on abc.com I know that.)

What I am trying to do is populate an email contact field from another (related) form.

Example my "Contacts" form for has:

abc.com, joe@abc.com
123.com, 123@gmail.com

So the goal is:

When a mail arrives and the "source" field reads "abc.com" populate the email field with joe@abc.com (So I can send the notification to joe@abc.com)
When a mail arrives and the "source" field reads "123.com" populate the email field with 123@gmail.com (So I send the notification to 123@gmail.com)

In a perfect world we could pass the email address in the HTML from data but that is impracticable so I have to do a lookup on my end.

I have managed to set up a relationship between the forms so I can see the email address in a pull down but I'm trying to set that via the contents of the source field. (like a relational database)

Pointers, links or tips appreciated.

EDIT: typo in subject makes it even worse!