Moneyguy1 - SMTP is the protocol used by mail clients and servers to send mail from the client (Outlook, Thunderbird, etc) to the server (your ISPs outgoing mail server) to another server (recipients ISP). If said recipient doesn't exists, a bounce message should be sent - something like "unknown recipient" or similar. Try sending mail with a regular client to
thisdoesnotexist@gruv.org - it will bounce. If it doesn't bounce, then it means that on my server - I own gruv.org - I've configured some action to happen when that address gets mail - either deliver it to another account or make it disappear and don't send anything back to the originating server.
Now if I wanted to make sure your address was good, I'd include an image that had a URL associated with it in the message - like sending a webpage, HTML mail can pull in remote images. So I include an image that lives at
www.gruv.org/spamimages/moneyguy1ataoldotcom.gif . Now this doesn't exist, but since I only sent it to you (I haven't actually), all I have to do later is look at my web server access logs and error logs and see which addresses show up looking for an image - moneyguy1ataoldotcom in this example.
Of course, this still won't work if you've set your mail clients to not display HTML mail, or if it does allow HTML mail, it won't display remote images (so it won't ask my server for the pic, etc).
Summary - don't worry about it. It is simply assumed that all messages get thru, and only 0.1% of the people that get it will be scammed/affected/buy the fake viagra/etc. Of course, when you send 10,000,000,000 mails, that 0.1% can still pay off big time.