Trying to work with the big sites to get whitelisted is one way to fight this. Bogus registration emails are rarely the problem - usually, it's notification emails. Members sign up for new post notifications in various forums and threads, and then don't bother to unsubscribe. Instead, they report them as spam. Lazy morons? Yes. Problem for community sites? Absolutely.

Innocent Joe gets an activation email

Please clarify what you mean by "activation email".

Is this a notification that the account is set-up and ready to use, or is it a request for verification?

If it is a request for verification, (typically by clicking on a link containing a code) then it should clearly state that "somebody" signed-up at your forum with this email address, and it might have been a mistake. This should make it clear that the message is not spam. It should say that if this is not the person that signed-up, they need do nothing, and the account will not be activated, and they will receive no more email from you.

I would also make sure that there is a response address - even though you don't require a response to delete the mail. Nothing gets some users blood boiling more than an email saying "do not respond to this email, automated, blah, blah, blah".

It is is NOT a request for verification, why isn't it? Nobody should be activating accounts without a response to an email these days. If you require verification, your site is useless to spammers. (But unfortunately, spammers don't seem very concerned about wasted effort...)

@rogerd: I'd consider that a legend - or has it ever been proven? I've *heard* as well that that may be the cause, but I have my doubts. Thinking about it myself, I would never flag notifications by a forum I post in as spam (simply because it's *not* more convenient than unsubscribing - well ok, webmasterworld may be an unfortunate exception). The emails by a forum I've never been to I would, though, especially if I receive more than one of those from different sites.

@jtara: Sure, those are requests for verification. Again, here I'm only judging by my own approach: when I get spam by a site I've never been to, I don't care if it says that "someone" signed me up and that it all may be a mistake (isn't that what spam mails always say?). If it looks like spam and feels like spam, I'll tag it, no matter what the exact wording.

Today I received a real spam mail that made it past my filter. It contains the access data for a site I've never been to. Seriously, how are people supposed to tell the difference? It's really an OLD, OLD scam technique.