Clarification on Leads, SQL & Pipeline Attribution from Email Marketing

We currently have 50K contacts whose Original Traffic Source is marked as Offline Sources. However, some of these contacts later converted and closed deals through Email Marketing efforts.

I would like to understand how many Leads, SQLs, and Pipeline have been generated specifically through Email Marketing. Since the Latest Traffic Source property is not working in this case, what would be the best way to accurately measure and attribute this?