Well first of all, in an email you would not have an address and the sender at the beginning of the email as it is usually located at the bottom. Also emails tend to have more smaller paragraphs and generally be shorter than letters. This is because in modern times e-mails are often read using devices such as a smartphone and reading giant paragraphs would be difficult on a smartphone. These are just some of the features that help you find out if something is an email.