When accusing domain owners of spam, don't forget that anyone can spam thousands of blog comments for $5 using any domain (think negative seo). For example, I really doubt that Steve Pavlina (#3 in the spam domains list) would spam his own domain. Most probably some competitor wants to take his domain out of the google index.

    Those were the top domains used by spammers:
     16514 gmail.com
     7300 hotmail.com
     3267 yahoo.com

How come gmail is so popular with spammers?

In terms of countries (based on the IP address of the spammer):

#1 USA (30%)

#2 China (20%)

#3 Russia (6%)

#4 Brazil (4%)

IP addresses aren't reliable, especially when considering spammers will know IP address geolocation will be used. It may be safe to assume a vast majority of US IP addresses will actually be proxies.