How does MageMail's email capture work?
The way that most checkouts work with Magento (Core Checkout, One Step Checkout, and others) is that the email address isn't saved to the quote until far along in the checkout process, sometimes not until the shipping method is selected.
So if a customer abandons before getting there, their email address isn't saved, even though they have already entered it in the checkout.
A benefit of MageMail is that it checks for email addresses entered in and saves them to the quote right away. It does this by looking at the `.validate-email` class. This works for both customers with registered accounts and guest customers.
If for some reason, the email capture needs to be disabled, that can be done under Promotions > MageMail Connector > MageMail Advanced Settings > Should Capture Email: