An anonymous user can not use our mail servers to send out mail unless there is a user set up, and they have the username and pw for it. However, as you know, anyone can send out emails from their own smtp server, setting the "from address" to any email address they like. There is nothing that we can do to prevent this, since they are using their own smtp servers to send the email.
In a similar manner, there are also viruses that have a built-in SMTP engine, running as if they are a "mini mail server" of their own. The virus may send out messages with the TO and FROM addresses both pulled from the address book, making it difficult to determine the exact sender of the message.
In both of these cases, you can look at the header information of the e-mail address to determine the actual sender's IP address, and sometimes their computer name (although both of these can be "spoofed" or faked by crafty hackers). You can do a "whois" on their IP, and report them to their ISP.
Article ID: 233, Created On: 3/28/2003, Modified: 5/26/2009