Could it be that a lot of people that haven't been on the forum for ages and forgot their passwords had gotten a message from Otto and wanted to get on the forum to see what it was?
Eva, this seems like a good theory.
I don't think this guy emailed anyone directly (most likely no email got disclosed).
The email people got was an automated email from the system telling them they received a message from otto. None of the content in the email was added by this user.
I'm not sure what happened, but all I can guess is that the system saw it was a message to too many people and didn't send it to anyone, but still sent the email notification as if it was sent.