The initial message might be sent to 10 people. Those 10 each send to 10 and create 100 messages. Those 100 each send to 10 and create 1000. Some chain mail messages claim that they are not illegal and go on to include what is claimed as proof. Do not believe them.
Chain mail violates federal law.
So do Pyramid scheme messages. A pyramid scheme calls for escalating numbers to send money, usually small amounts, to others, with the expectation that a large amount of money will come to them. Do not believe them. Pyramid schemes are NOT legal. A virus hoax is a type of chain mail.