Spam increased by 1200%

Within the last three days the mails denied because of an unknown recipient has dramatically increased on my servers: From 1-2 each 5 hours to up to 100 last Wednesday (08.08.) Since then it's decreasing slowly down to 30 and within the last 10 minutes it decreased down to 10 again..
That does no harm to me but it's interesting.. Does anyone else see this on his servers..?