Generally, SPAM comes from giving out your email address somewhere on the internet. E-mail addresses can be harvested from chat rooms, newsgroup posts, signup forms, and any webpage that they are posted on (such as personal homepages). SPAM can also be generated by computers infected with viruses, and sent out to people whose addresses are stored on those computers in address books or other places.
Spamming happens when your mail address get disclosed to the spammers. We recommend you to do the following measures to decrease spamming to your mail accounts :
1) Do not publicly disclose your email ids in forums, websites, newsgroups, blog comments and chats.
2) When you are subscribing to anything important, do not select something like: "YES, I want to be contacted by select third parties concerning products I might be interested in." If selected, you are giving consent to receive mails from them.
3) Avoid replying to spam/unsolicited mails.