Someone's sending from my email address! How do I stop them?!

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Summary: Email spoofing is rampant. Spammers often send email that looks like it came from you. And there's little you can do about it.

You're minding your own business and one day you get email from someone you've never heard of and they're asking you to stop sending them email. Or worse, they're angry. Or worse yet, they accuse you of sending them a virus! But you don't know them, you've never heard of them, and you know you've never sent them email.

Welcome to the world of viruses where you can get the blame for someone else's infection. And there's worse news to come.

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Before I get to that, there is always a small possibility that your email account has been compromised. The solution there is simple: change your password immediately. That should prevent someone who's using your account for malicious purposes from continuing, assuming you've chosen a good password.

But these days that's not the most common cause for the situation I've described, viruses are. And what's worse, there's almost nothing you can do.

The MyDoom/Novarg virus currently running rampant is a great example. The virus infects someone's machine and then looks in the email address book on that machine and emails a copy of itself to everyone it finds. What it also does is forge the "From:" address for the email that it sends. What does it use to forge the address? Why, the addresses in the address book, of course. So the infected machine will send email to everyone in the address book, looking as if it was sent by other people in that address book even though it was not.

Let's use a concrete example: Peter's machine gets infected with the MyDoom virus. In his address book are entries for friends Paul, and Mary. Paul and Mary have never met, have never exchanged email, and do not know each other - they each just know Peter. The virus on Peter's machine will send email with the virus to Paul looking like it came from Mary. Paul may wonder who the heck this Mary person is and why she's sending him a virus, but she was never involved.

If you're in Mary's place, you can see that it would be frustrating to be accused of something that you had nothing to do with and have no control over.

For the record, your email address may end up in the address books of people you don't know as well. Various email programs will automatically hold on to additional email addresses that were included on email you received or possibly from email that was forwarded. Viruses have also been known to use other sources of email addresses or even forward them around as the virus spreads. What that means is that the simple "friend of a friend" example I used with Peter, Paul and Mary, while simple and certainly possible is not the only way your email could show up as a forged "from" line.

What's important here is simply this: one way or another email viruses lie about who sent them.

If someone accuses you of sending a virus-laden email, and you are positive you did not, then you have very little recourse other than trying to educate them about how viruses work. Point them at this article if you like. But be clear: you're not necessarily infected nor is the person who received the mail claiming to be from you. It's some third party who is. (And identifying that third party is difficult - this is why virus writers use this technique.)

And of course be sure that you're not going to get infected yourself: don't open attachments from people you don't know and make sure you have an up-to-date virus checker and virus definitions file. I have recommendations for virus scanning software here.

Related:

• Ask Leo! - Someone has stolen my email account. What can I do to get it back?.

• Ask Leo! - How do I keep my computer safe on the internet?

• Ask Leo! - How do I keep my computer safe from viruses?

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• Amazon.com - Best Selling books about PC Security

Article 145 | Posted January 27, 2004