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

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 that you've chosen a good password. (If you find that your account has indeed been compromised, you may want to change more than just your password.)

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

The typical virus that does this infects someone's machine and then sends spam, lots of spam, forging the "From:" address for the email that it sends. What addresses does it use to forge the address? Any address that it can find and probably at random. That could be other email addresses that it's sending to, email addresses fed to it by a botnet, or perhaps even the addresses in the address book of the infected machine. The infected machine will send email to everyone that it can, looking as if it was sent by other people.

And you could be one of those "other people", even if you had nothing to do with any of this.

Peter, Paul, and Mary's Email

Let's use a concrete example: Peter's machine gets infected with a virus. In his address book are entries for his 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 who you don't know as well. Various email programs will automatically hold on to additional email addresses that were included on email that 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 that I used with Peter, Paul, and Mary, while simple and certainly possible, is not the only way that 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.

There's Little You Can Do

If someone accuses you of sending a virus-laden email and you are positive that you did not do it, then you have very little recourse other than to try 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.)

In other words, there's nothing that you can do.

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 that you have an up-to-date virus checker and virus definitions file. My recommendations are here: What Security Software do you recommend?