When you save a contact resource with an email address a 'Confirm' button appears at the bottom of the form.
Clicking the confirm button will send an email like the one in the image at the top of this post.
When your contact clicks on the 'Confirm Details' button in the email they'll be taken to a secure page on this website that will allow them to update any details that are missing or incorrect.
In this way, you can be sure you're using the most up-to-date information.