Spam: E-mail advertisements, publications or other materials sent to users who do not take the initiative to ask for instructions; mails without clear communication methods, senders, return addresses, etc; using China Telecom's network to engage in acts that violate other ISP's security policies or service terms; and other mails that are expected to lead to complaints. Based on the above definitions, which of the following is not a spam? ['an advertisement email introducing the latest skin care products but dont know where it from.', 'an enthusiastic letter of praise but no clear sender.', 'an email from a known contact but with a virus in it.', 'an email with winning information from an unknown website.']