Spam is that one annoying thing which everyone gets in his or her mailbox, it happens everyday, to almost everyone. So how does spam find its way to your mailbox? Its quite simple, the people doing the spamming acquire your email address with certain methods. These methods can be harvesting, which is the process of scouring the internet looking for email address, spammers also buy email address lists from people who are willing to sell peoples emails, and the most popular way spammers get your email is when you sign up to newsletters or to a service.
Spam can be blocked though, by introducing a middleman, in this case a hosted spam filtering service. The spam filter service will receive the email destined to you, it goes through the email, looking for any hints of spam, and if the email is clean it gets forwarded to the recipient.
The spam filter service is very configurable, with white lists and blacklists. This way you can allow some certain emails through and at the same time you can block a lot useless spam. At configurable times, you, the recipient can get detailed reports as to what has been blocked by the service, and you can release the blocked emails, to your inbox, should you want to.