Why am I suddenly getting spam?

People often wonder just how spammers get their email address in the first place. There are many, many possibilities. Here are just a few:

• You posted your email address in public on a web site. For example, let's say you mention your email address in a comment you post on someone's blog. Spammers regularly scour the internet looking for anything that looks like an email address, and they start sending spam to it. I think this is possibly the most common form of direct email address harvesting today.

• You posted your email address in public on a newsgroup or forum. I did this years ago by mistake (with my wife's email address, no less). Usenet and many forum management packages still display publicly whatever email address you give them. Many do not, so it's important to know the difference. (The good news is that most discussion forum software these days is better behaved. Usenet is still a mess, though. Fortunately it's slowly falling out of favor.)

• You joined a mailing list that keeps public archives on the web. These archives often include the full mail header of each message, including your email address - ripe for harvesting by spammers.

• A friend of yours forwarded an email of yours without removing your email address from it. I see this all the time with forwarded humor - people hit forward and then fail to take the time to remove all the email headers from the body of the message. You've seen it too, I expect; email you have to page down multiple times to skip all the email headers before you actually get to see the body. That email frequently ends up getting forwarded (unintentionally, I might add) to people who then scan the email body for anything that looks like an email address.

• You bought something from a less-than-reputable retailer or service. Some companies will sell or rent the list of email addresses they've collected. Once they give the list of email addresses to someone else, all bets are off since that other person could do anything with that list, including selling it to spammers.

• You sent or received email. I know this sounds silly - of course you sent or received email! That's what your email address is for, after all. The problem is that email, including the addresses it goes to and comes from, is sent in plain text. Since email is sometimes routed from server to server on its way to its final destination, it's possible that a spammer can "sniff" the traffic and harvest any email addresses found.

• They guessed. Seriously, that's why email names like "firstname" @ any domain name are spam targets. Spammers often simply try sending email to every email name they've ever encountered at every domain they've ever seen. I can tell you that "leo @ whatever" gets lots of spam. This is an extremely common technique, as anyone who's looked at mail server logs can tell you.

There are probably many more ways that spammers can get your email address; those are just a few I could think of quickly. You'll note that most are not in your control at all. Spam is, unfortunately, inevitable.

So, what can you do?

Well, I can tell you that flagging specific email addresses that you get email From: as spammers simply won't work. Spammers now regularly fake or "spoof" the email address that appears in the "From:" field. It didn't come from there at all. And they keep picking new ones at random.

The bottom line is that using the "From:" field as a way to stop spam is nearly useless in many cases. Sometimes there are specific spammers that do send from a single address, but they're infrequent. So I wouldn't bother blacklisting an email address until or unless you specifically get multiple spam messages from them.

The real answer is that there is no answer. There is no single solution or combination of solutions that will make spam stop without also preventing legitimate messages from getting through.

You can sometimes reduce the amount of spam by:

• Making sure your ISP and/or your email program has a spam filter, and that the filter is enabled. (GMail is currently very popular for their spam filtering abilities.)

• Periodically changing your email address. This can be painful since all your contacts need to update their information in order to contact you.

• Use a challenge/response system so only people that prove they're human can email you. I do not recommend this solution (you will miss email you wanted), but include it for completeness since there are people who swear by it.

I have three spam filters in place: since I run my own mail server, two are implemented there (and are frequently also implemented by ISPs), and I have the spam filter in Thunderbird.

And still some spam makes it through.

My next line of defense?

The Delete key. Apply it liberally, and get on with your life.