Sometimes, when I'm feeling uncharitable, I think that people on the Internet are stupid. This is not a fair criticism, as there's clearly lots of clever people out there. But I sometimes fall into the trap of thinking this because of some of the email I get from people who are either misguided, poor typists, or simply, clearly don't understand how this stuff works.
I currently hold a GMail account. Because I got in early, as techgeeks are want to do, I got
email@example.com. As it happens, I don't even use it for email correspondance -- even though Google are quite happy to offer me (at time of writing) 7.5GB of free space for such.
However, despite the fact that I don't advertise this email address, and Google's spam filters are pretty good, I get a steady stream of email, mostly generated by, or on behalf of, people I sometimes uncharitably refer to as idiots.
For example, there was the case several years ago when a chap called Dan, who lived in Texas, filled in a signed a legal document relating to the purchase of a 450,000 USD ranch -- near San Antonio, if memory serves. On page 7 of this document, he dutifully filled in his name, residence, and my email address, then sent it off to a property lawyer -- who then dutifully scanned the paper document in, and emailed me a copy.
More recently, I've been finding that my GMail account has been signed up for all kinds of different services -- including a full year's subscription to Microsoft's XBox Live Gold, to Facebook and similar social networking services (multiple times), to something about golfing, to something about wine-tasting in the Easter United States, and so on. And that's not counting emails I get from airlines, schools, hospitals, and other institutions trying to contact one of their number about some internal administrivia. Replying to these and explaining that, no, Dr. such-and-such does not own this email address and they probably want to check more carefully the email address they've been handed is tedious; some point soon, I'm going to stop bothering.
People mistyping email addresses -- even if you ask them to type it in twice on a web-form -- is going to keep happening. Sometimes this will be because they are deluded, persistently bad at typing, or simply don't understand that their email address isn't something they get a free choice in selecting. (In one comedic example, one young College student in the US made my GMail address the backup contact for her new GMail account -- and then started signing me up to all sorts of College-related services.)
At least with humans, they can have the problem explained, and they'll stop doing it. (Until the next one comes along.) Robots which send bulk email cannot be reasoned with quite so clearly -- it's rare for autogenerated email to have a "Received this email in error?" clause in their logic. People writing web robots that subscribe email addresses to services should take more care. Specifically, they should add a repudiation link -- to say, in effect, "You have signed up the wrong email address." Some already do this; Facebook in particular handle this case quite well (though they probably shouldn't be providing login-equivalent URLs to email addresses that haven't been verified.)
But it would be nice to see more of this kind of thing. It's in their best interests, really -- otherwise the cumulative 'This is spam' reports received by the major web email provider might become significant enough that they'll get grey- or black-listed. It'll also save them a lot of time and effort as and when they try to correlate their subscriber base with email lists from other sources..