Email business letters use the same medium as the basic business message.
But they're a more complex form of communication, and don't really fit the message format. Unless the letter is particularly brief, a standard business letter should be used as an attachment.
The most useful thing about a business letter as an attachment is quality. Using a standard macro, things like references and other information which would normally be in a hard copy letter are naturally included.
Some people use PDFs of their business letterhead or other documents as attachments, which protects the document from tampering and gives good presentation.
Note: It's standard practice to refer to attachments in the message text, and include any notes for the reader's reference.
ExampleTo: Ted Rice
I'm happy to say we've just introduced a new menu which may be of interest to you and your guests. The attached document is a whole range of desserts, which wasn't on our menu when you made your orders. Do have a look and see if there's anything you'd like included.