Any e-mail address can be used to send a e-mail but you can’t use the same ip as someone else but you can use an anonnymous e-mailer that uses proxies.

Actually, that all depends on what ISP you use. AOL uses a pool of IP’s that switch from user to user… that is why many aol users have issues with certain forums and websites. If a certain user who was currently using that ip managed to get it banned from the site, any other user who happens to use that IP to log onto the same site will not be allowed to. So technically you can use someone elses IP. 😉