According to this article at The Register:

The United States continues to lead as the main origin of spam, with nearly 55.69 per cent of all spam being sent from the U.S. South Korea (10.23 per cent), China (6.60 per cent), Brazil (3.35 per cent) and Canada (3.08 per cent) are other common sources of spam.

Well, at least the good old U.S. is still holding fast in the number one spot.

Read the article for more details about who is cluttering up your mailbox.