Spam emails - what just happened?

[#1]

Anyone else just experienced a brief, but sustained spam attack. I've just received, in the last 20 mins, over 50 spam emails, most of the 'message undeliverable...' type. Things seem to have calmed down now though...

[#2]

http://en.wikipedia.org/wiki/Backscatter#Backscatter_of_email_spam
It should subside within a few hours/days
B.