The price increases prompted an increase of "underground IDs" (known as 'UG's for shorthand)—where multiple users shared a single account that they turned into private bulletin boards by using emails that were returned (and therefore, not billed) due to invalid email addresses. Those invalid addresses were the simple names of the person or people for whom the messages were intended. When those people signed in and checked the email, they would find "returned" messages with their names. They would then "send" a reply by typing the name of the first sender, which would also be returned. When that person logged on next, they would see their message, and the cycle would repeat.