ESTA: How to confirm your email with the 4-digit code?

Since November 2019, there's a new feature on the ESTA form: you must confirm your email address by receiving a code. Here's how to do it.

How to confirm your email address

Since an update in November 2019, the ESTA USA form now requires you to confirm your email address using a 4-digit or letter verification code.
Go to the bottom of the first page, labeled “Applicant Information.” After entering your passport details and nationality, you’ll need to provide your email address. The form will ask you to confirm it by entering it a second time.
That’s not all. When you click “Next” to proceed to the rest of the form, you’ll need to verify your email. Here’s how it works: a 4-digit code will be sent to your email, and you’ll enter it on screen to continue.
If you want to use a different email address, click “change email address.”

When the system first launched, several members of our New York forum and Destination New York Facebook group had trouble receiving the verification code, even though their email addresses were correct.
Try switching to a different web browser. The ESTA form works better with Firefox than with other browsers.
Another solution that worked for one of our members: instead of clicking “Next” at the end of the first page, click “Save and Exit.” This lets you save your progress and continue the form later. Using this older function, the verification email was sent without problems.
Once you’ve completed this step, you can finish filling out the form. Here’s an article with my guide to completing the ESTA form.