I just found my activation email sitting in the Googlemail spam folder. I don't know why it was determined to be spam, but if other people are experiencing similar problems with other spam filters, it mgiht be worth looking into. On the other hand, if it were that easy, all the spammers would do it, so...?

Gmail's spam filter isn't flawless. I told it to mark that message as not-spam, and gmail hasn't incorrectly flagged any more messages from clamwin forums since.