But you should also keep track of the email address that they signed up with in a case sensitive fashion. Any time you send an email to them, be sure to send it with that original casing. This allows the email server to handle it however it feels like it needs to. So even though the person may always be signing in to your site with steve@example.com, if they signed up as Steve@example.com, you'll always send email to Steve@example.com, just to be safe.