Mailer Daemon is not on your machine. It is the name used by most commercial mail servers to return mail to you when it can not be delivered. If you read the messages it will tell you which messages and where you sent them to. It could be they are telling you one of the addresses you are sending to does not exist, the user's mailbox is full, or that you are on a blacklist because the server thinks you are sending spam. If you do not read them you can not tell what they mean. It could be that another person has managed to send out from somewhere else using your email address as the sender address. This is often because someone who has your address in their address book has had a virus infection on their machine and the virus sent ALL their email addresses to the virus originator. There is not much you can do about that unless there is a common part of the message, like they were all sent from the same server, where the yahoo admin may be able to stop it. For total peace of mind, download the free scanner from http://malwarebytes.org and run a full scan. McAfee has been known to mis things and an infection on your machine might be sending out using your account at Yahoo, but it is unlikely.