Hi Dan

That is a good question.

A guest checkout can still receive the cart abandon email if the customer got far enough to reach the Checkout page and entered their email address without completing checkout.

The cart is considered abandoned if the timeout exceeds the Cart abandon timeout value set under Configuration > Cart settings. 

For more information, please see this topic.

I hope this helps.

Thanks for the info. So on the checkout page, if the user enters details with their email address, does it do some updating automatically without the user attempting to checkout?

Yes, there are several form field triggers that can capture the user details without the user necessarily needing to hit the place order button.