How do I enable 2 factor newsletter signup verification?

You can enable 2-factor authentication within Magento by going to your Promotions > MageMail Connector > Newsletter and saying Yes to Need to Confirm
Then, you'll want to set the Newsletter Trigger > Filter > Newsletter Subscriber Filter to Newsletter Subscribers Only, so it'll catch the newly authorized subscriptions.

Once customers click the authorization email, they'll appear in Pending and will send out!