Business Process checking for existence of contact

[RIchard Horton]

I'm wanting to do a decision within a business process within HSM where I use a custom value that has been extracted into a variable, and check whether that value (which will be an email address) corresponds with an existing contact. Anyone in a position to point me in the right direction ? Assuming that I can do that, the next question is whether there would be a way of unarchiving a contact if the response is that they have been archived. I'm less worried about that though - happy to start with existence/non-existence. 

Thanks

Richard