Why Does a Email Form Say “Invalid Email” as I’m Typing the Address?

Episode 1836 (15:13)

Ed from United Kingdom
Email

Ed is having a weird experience when he types in an email address and gets "invalid email address" after just a few characters. But if he pastes in the email, it's fine. Leo says that there's code in an email form that is constantly checking for legitimate emails. It should do it after you type it in, but it's doing it on each letter, and that's clearly a bug. But if you keep typing, it should just nag you until you're gone inputting the email address. So it's either a bug or just plain bad form design.