Use a valid and active email address in your reply to address. Lead Liaison allows marketing to send email on behalf of sales. For example, if John Smith, john@company.com, is the lead owner of Susan Que then marketing can send lead nurturing campaigns or one-off marketing messages to contacts like Susan as if they came from john@company.com. This takes the burden off of sales to stay in touch with their prospects giving them more time to focus on closing deals.