Workflow that updates "New E-mail Field"

Hi,

I would like to create a Workflow, if a Case is new and an email has been incoming (A case is created automatically), I would like a custom field "New E-mail" to be updated that i've created. If we don't manage to answer to the Case, before they send another one, I would like to update the field. 

How would you achieve this?

Thank you!