If it's from social media or networking, it can be assumed that it's an email provided by a real person through a landing page/registration, which would mean a verification can come later on (after a few months to make sure people are still at the email given). If it is a list where an opt in is not clearly identified, you may have to verify earlier that the emails are real. It's more about timing and making a call on how the emails were first provided. Hope that helps.