getting users to send you emails without them knowing is akin to virus writing. yes, you can do it on the client, but that's not wot permissable for the reason alluded to above.
therefore, you're better doing this on the server, whereby a a remote script is called each time a user logs on, and thus the server sends you the email not the user's machine.
depending on wot sort of server you're using, you're better to check out the PHP or ASP forums for the answer you seek.
PM a Mod and they'll move this thread for you. don't cross post, that'll just waste time.