IMVHO it's a matter of spammers already sending mass emails to every likely username in a given domain, even if that username isn't currently in use. they may get the list of likely usernames from software and online dictionaries, by a devious and cunning process. then when some unlucky user activates one of the names on the spammers' lists in what the user intended to be a "dedicated email address", the spam that was already being sent to that username starts to appear immediately.