Reminder for Incomplete Registration (and continue...)
You could implement this with Webform-CiviCRM integration, assuming your registration page is a webform.  If it's not, it's still doable, but would take a bit of custom code.

First, set up a "Step 1" webform that just collects email.  On submit, redirect to your "step 2" webform (where registration actually happens), passing the email address in the submit URL.  Also add the person to an "incomplete registration" group in CiviCRM.

If event registration is a webform:

Next, grab the email from the URL on the step 2 webform. See here for detailed instructions.

On submit, use the Rules module to remove the person from the "incomplete registration" group.

If event registration is a CiviEvent form

Write a small custom module to pull in the email address on to your CiviEvent form.  Details on the technique are here.  Alternatively, skip the custom code, but then the email address won't be prepopulated.

Use CiviRules to remove the person from the "incomplete registration" group.