I got spam with my email as return address

I know that it's easy to spoof an email address. However, it really looks like this email came from my address. How can I tell for sure? Thanks,

I know that it's easy to spoof an email address.

OK.
If you know the above to be true, you also know that it is an often used vector for promoting spam.
And by extension, you would know that some efforts will be quite sophisticated, while others will be somewhat amateurish.

What you need to keep in mind is that email is "communicated openly" and anyone with the requisite skill can find opportunity to intercept, read and corrupt an email, including corrupting the envelope "wrapper" addressing information.

How do you know that it didn't originate intentionally from your email address?
Well, you know… if you didn't write it.

So if it appears, and you didn't write it, something else is afoot.

Your address could have been harvested from your own machine, if your machine is infected with malware.
Your address could have been harvested from someone else's machine if their machine is infected.
Your address could have been harvested from a sign-up/ login on a site that has a poor privacy policy or is otherwise unscrupulous or is otherwise poorly scripted to protect users.

Here's an "old" source for general information about security and reporting.
http://www.cert.org/tech_tips/email_spoofing.html
"Old" is not necessarily a bad thing, since SMTP (simple mail transfer protocol) has been around in its present form for a long time.

Here's a source that gives you a bit more "step-by-step" information about investigating the Full Internet Header information used in routing email.
http://www.scribd.com/doc/13055/Email-spoo…ith-the-problem

And in the US there is a Law - CAN-SPAM (Controlling the Assault of Non-Solicited Pornography and Marketing Act)
read here about that law and how it might or might not help:
http://www.ftc.gov/bcp/edu/pubs/business/e…erce/bus61.shtm

Best Regards

Thanks for the reply and tips. It appears that the emails, are simply spoofing the addresses in the sent from field. Evidently several customers of my ISP are having this issue. Thanks again, you can close this thread.