I am receiving email spam.

Addressing the question of Spam, BrickLink respects the privacy of our members. BrickLink does not send you unsolicited e-mail or sell your email address to 3rd parties and will never give out your address and email address to anyone except to those registered BrickLink members with whom you do business. Your personal information including any or all of your data can be however provided to verified law enforcement who are investigating an illegal activity or a crime.
Now, how could Spam get into your Inbox if you don't use it anywhere else? There are a couple of reasons, such as mail relaying. Mail relaying is sending mail to a user that is not local to the server you have connected to. Another reason could be dictionary attacks. Dictionary attacks happen when spammers connect to your server and just trying random address or even stepping through the alphabet. Some servers will block the sending server after X number of failures, some set max recipients per message and others set a delay between recipients to slow down the spammer, causing them to go elsewhere.
Here is a handy page describing the ways that Spammers can get hold of your e-mail address: http://www.faqs.org/faqs/net-abuse-faq/harvest/.