I wouldn't believe it myself, until it happened to me. I've run small mail servers for various domains for years with only "average" amounts of spam. Then one time I picked up a personal domain of my handle (not this one) which I'd wanted for years and I discovered it finally was free. The moment I setup an mx for that domain I got a firehose of spam coming in to it. In fact it was so bad I had to unreg the mx and leave it for another year before it was worth doing anything email-wise with the domain. Just the bounce processing was driving cpu and bandwidth more than I cared to deal with, even if it wasn't receiving any local mail.