The sending of chain letters is against yoyo regulations and Computer Centre regulations. Sending such mail can get your yoyo account suspended and you can face other action by the Computer Centre.
Chain letters are stupid, wasteful of resources and in some cases, illegal.
This is any letter that invites you to send it to multiple people who will be asked to do the same thing. They often threaten bad luck if you don't do it. This is rubish. All that happens is that you quickly clog up the mail system and this costs people real money all over the world. This includes any mailing that invites people to forward it to other people and so on.