I once got bombarded with the "undeliverable" emails.

As far as I can remember, it turned out that a fair number of others using the same ISP (possibly Red Fox Hosting) were also affected and the ISP put it down to an assault on their email system which was getting my say 100s of bounce emails per hour X multiple email accounts. I think the ISPs response was to suspend the email for the affected addresses until things died down and that it was about a month before these email accounts were restored.