Unsolicited e-mails are known as 'spam'. Spam is the junk mail of the information age. Sent in bulk, it clogs up your inbox instead of your letterbox and forces you to clean it out daily. Senders of spam are known as 'spammers'.
Spammers are chancers.
They fire off a million e-mails in the hope of receiving a few replies. As it is incredibly cheap to produce, if a spammer receives just one credit card number in reply he will recoup his investment. Simply because it is very hard to trace them. These unprotected mailservers are a remnant from the days when the internet was all about sharing your resources. Nowadays, many mailservers have security measures to prevent such abuse.