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The subject line from the e-mail that just landed in your inbox indicates the message was returned because it could not be delivered. Upon closer inspection, the message — hawking cheap designer watches — doesn’t look like any message you’ve ever sent. What’s going on here? Is there a ghost in your machine? Has it been commandeered by criminals and enslaved as a spam zombie?

These are questions that I’ve been asked many times, and while I may not have a definitive answer, there may be a simple explanation for what’s really happening with your e-mail account.

Spammers blast their junk mails out to millions of e-mail addresses that are usually purchased in bulk and/or scraped from various Web sites and forums. But some spammers also use those lists to fake or “spoof” the address in the “From:” field of each e-mail sent. That means that if they spoof your e-mail address in a message sent to an address that is no longer active, your inbox will receive the automated bounce-back reply explaining that the message could not be delivered.

Using regular old snail mail as an example may help readers best conceptualize what’s happening in this case. Let’s say Alice sends a letter through the U.S. Postal Service to Bob, but instead of writing her own return address on the back of the envelope, Alice puts Charlie’s physical address there. If Bob no longer lives at the address to which Alice sent the letter (and the Post Office has no records of Bob’s forwarding address), the Post Office will return the letter to Charlie. While Charlie didn’t send the letter, the Post Office doesn’t really know that - it only knows that Charlie’s is the address listed as that of the sender.

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