It could be that a spammer has 'borrowed' your email address and sent out loads of email to random addresses, many of which don't exist, resulting in you getting notifications about undelivered mail (or something similar).
Myself, and it appears countless other customers are receiving unsolicited emails, I must have had over 500 in the last couple of hours. It's very annoying but not helped by people replying to them and thereby increasing the number of mails being received. It seems that virgin are aware of the situation so am having to sit tight and keep on deleting.